This is not something you solicit advice and guidance for from a blog or chatroom.

This will be a rather difficult computation and install and those that have the ability to do it will likely charge a nice fee for the service. If your cabinets are already installed, it may not even be possible or practical.
If installed, I'd suggest you go with a stainless or black or white factory front to match your other appliances.

Re: your questions

Only YOUR countertop gap is dimension is pertinent and is determined by the cabinet maker.

Dimension of top of cabinets (backer) to bottom is again determined by the cabinet maker. It is nominally 30", but the EXACT measure rarely exactly 30" .

Backers are typically not use with Bosch DWs, they are designed to be used with the "door" panel from the cabinet maker and is typically 3/4". They can vary from 5/8" to 1" though.

If you're insistent and don't have access to design help locally I'd get the manuf. to make an oversize door/drawer and rabet out the frame portion that would normally be exposed.

I forgot to mention that the cabinet company can only make regular size drawers/doors for 24" cabinet. (Can't make sizes needed for the rabet idea.)

How do I determine my countertop gap?

My cabinets are standard 30". Kitchen designer made a panel bigger than 30" which doesn't fit, but kitchen designer is claiming no error.

Should I use the thick 3/4 backer with painted edges or thin backer with unfinished edges?

By the way, do you think I should pay twice when designer mismeasured? (Designer mismeasured refrigerator panel, so this is not the first error.)

I don't think you should pay twice, for what it's worth. How can the KD claim no error if the thing doesn't actually fit? That's a keen trick. ;-) Is it not possible to just cut the panel down a bit?

Do you have the Bosch yet? Mine came with a paper template for the panel showing where to attach the fake drawer front etc. We made our own and it's a full inset shaker door, no drawer, so I can't quite help you there. The panel on my Bosch screws in along the sides, top & bottom, so the thickness is only important in those places. Do you have a photo of your exact door & drawer style?

Terrible picture, but the yellow dots are where the screws go. You just need your panel to have its main strength around the perimeter:

Yes, I have the Bosch.

No, there is no template.

No, I can't shave it down because of the detail on the edges of the cabinet and too much would be cut off.

I am glad you agree with me that I shouldn't have to pay. (Kitchen designer did not make me pay twice for the refrigerator panel mismeasure.)

Euro cabinetry is frameless. They don't have partial overlay there. You purchased a Euro DW. It really isn't designed to ever mimic a partial overlay door.

To kludge one together, you're gonna need more depth front to back in your installation. The 3/4 panel is supposed to be the door, but you're gonna have to use a 3/4 panel as a backer to a 3/4 door and drawer set onto it to mimic the overlay. THe problem will come if you don't have enough depth front to back to push your DW in further to get the doors to line up. And, the problem will also come from the weight. It's not designed to handle that much weight and it'll void the DW warranty to do what you want to do.

If you want to have your DW appear invisible, then take a look at the specs for the built in DWs and find one that uses a 1/4" panel to which you attach doors. Or change to full overlay cabinetry. ;) Choosing a new DW is probably the cheaper route.

I don't know if it is past the time to return the DW and get a refund and I did get it on 25% sale. Isn't Bosch supposed to be a good DW anyway?

I will have about 3 inches behind the DW.

Cabinetry is already ordered.

Should I use the thinner backer because of the weight (even though I have plenty of room behind)?

Some good news is the thinner backer panel with unfinished edge was ordered, so I may only need the drawer and door front.

unfinished edge is not good for a concealed dishwasher.

Live_wire is exactly right, the 3/4" backer is the panel, the drawer and door are appliques. They would be ordered as is they were ordinary door/drawer fronts for a regular 24" cabinet.
The Bosch will still work if recessed the extra 3/4" because they are so very shallow to begin with.
The Bosch instructions give the dimensions, but I'd make the flat panel 23 3/4" wide and 29 3/4" tall. The height can be further reduced along the bottom edge and touched-up on site without too much angst. The doors would by sized "typical" because you want them to match the rest of the kitchen, and they would be aligned into proper position by the installer, working on-site, with the backer mounted. (then take the backer off and attach the door from behind with screws).

So the 3/4 backer won't be too heavy? (KD thinks it is.)

Good point. The Bosch door springs are adjustable, either by changing which hole the spring goes into, or by substituting a heavier spring.
Casey

Does the Bosch come with a heavier spring?

Is it not a good idea to use the thin backer (already paid for and shipped) and paint the unfinished edge?

Also, is it better to get 3 doors to match next cabinet at end of run or better to get door/drawer to match most of the cabinet run?

Is there a weight limit on the specs? There was on the Miele.

I think the weight said 9-18 lbs.

Installer says I should go with 3/4 because thinner backer might warp.

However, thinner backer is already paid for and has arrived. I don't know if I can get another backer free. What do you think?

The edge of the thin (plywood?) backer is in a vulnerable position to getting snagged, warping (since there's nothing to support it. There is a metal channel that you could border it with, but it would show.
If the wood was poplar, painted, it just may be within the weight limits.
Casey

Weight limit 9-18 lbs.

Casey, do you think if I use the 3/4 it would still fit the weight requirements?

Where can I get a metal backer?

They sell short lengths of aluminum channel at HD, otherwise you need to find a metal fabricator shop.
I'm thinking of a c-channel with a 1/4" spread. It would be cut/sectioned to be folded to the perimeter of the thin plywood backer. If left open-ended (3-sided), the corners could be welded and finished, otherwise the seams will show more, but they can't wrap the plywood.
Casey