Kate - welcome to the sqft gardening forum, and method!

Being one of the online friends that said to go deeper, I have the following opinions for your situation.

Assuming your drainage problems have been completely corrected by the french drain system that farmer B put in, I'd stack 2 of you currently built boxes - with 2x2 supports screwed to the inside corners. If you went with 2x6 boards, this will give you a total depth of 11", and after the mix settles, should still provide between 8 and 9 inches of "soil" to grow in. Mel lives in Utah, which is a MUCH different climate than you and I live in. Trust me, your plants will appreciate the extra depth.

Don't worry about achieving the exact mix that Mel suggests....compost is the most important part, and many grow in nothing but compost. Just wait til you see my sweet potato harvest from growing in 100% compost, and you'll see the results.

EG

I've found that the recommended 6 inches dries out way too fast for my liking. I live in Utah, probably about 2 hours away from Mel, and I find myself watering it too much. I like 1-3 times per week, the less the better, and maybe that's my problem. I don't want to water every day :). I'm going to add another 6 inches to the box and it should be a lot better.

I didn't really take care of the SFG this year. I started it around May thinking that we wouldn't be able to have our regular garden for corn and potatoes/tomatoes and whatever else we decided to grow, so I tried the SFG. It turns out we had a new spot tilled for the garden and I neglected my SFG. I didn't even get a trellis up for the peas/beans/tomatoes!

I've had such a great crop of everything so far from both the garden and the SFG, even though the SFG was pretty neglected. I didn't quite fill up all the squares either, so that could have been why it was drying out fast.

I have probably harvested around 150 pounds of tomatoes, from 4 plants! These are all in the garden. My aunt told me at the beginning of the season that she would can any tomatoes that we didn't use and give us however many cans she wanted. I don't think she was expecting such a huge harvest...
I have some cherry tomatoes in the SFG, and since I didn't trellis them, they aren't very easy to pick. What I can reach has been more than I expected though.

Well that was completely random, but I hadn't updated my garden progress since I first planted the SFG.

I'm not thinking so. I have beds that are 6, 8, 10 and 12 inches deep. The ones in the 10-12 out pace the others incredibly.

Can it be done? Yes, but even though the expense is more to build and fill the larger beds, it's worth it in the long run.

I did 10 inches, but wish I went to 12.... only because the soil and vermiculite in the box end up lowering or compacting, however you want to call it. and now my soil level is a good four inches below the top line, so i'm really only at 4 inches.....

We had an exceptionally hot and dry summer here in Texas, and I think that my SFG would have done better if it had been deeper. I used 8" wood and filled it nearly to the top, and I followed the recommended watering schedule for the area. I think a deeper box would have been less likely to overheat and burn the plants to a crisp.

I would stack your 6" boxes on top of each other, to result in one deep box. Or if you want to keep those, try building another box at 10" or 12", and you can compare for yourself. Your 6" boxes will probably be fine for the fall and early spring garden, but not so great next summer.

I am not really a new gardener..just a dumb gardner! I am starting my first SFG. I had asked on another question if a botton was necessary for a SFG box. I receive some good kind advice that it should be ok. I am building my new SFG directly on top of some good garden soil. It is orginaly very sandy soil but now after some years of ammending, it is fairly good soil.(very good drainage) I am using 2 x 6's. I figure root crops such as
carrots would just deep going deep into the soil underneath once they grew beyound the Mel's Mix. If so, wouldn't this be at least desireable as deeper boxes?

I use 6 inch deep boxes in St Lucie County, Florida. That makes me the southeastern part of the state. I've had no problems with six inches, either in raised bed (meaning they had a bottom) or in on-ground boxes (no bottom). I typically water for about 5 minutes 3 mornings a week.

Take into account that my on-ground boxes are under the front eaves of my porch, so they only get rainwater if it's blowing rain. Otherwise, the only water they get is what I put on them. However, they also don't get the full heat of the midday sun, so I don't have problems with plants scorching in full sun like others might have.

I live in TN and raised tomatoes both cherry and regular, yellow squash, sweet peppers and cucumbers in a 4x4 6 inch deep raised garden last year. I watered every evening. Since it is drier in Florida you might want to water every morning. My only problem was keeping the deer and neighbors away. I gave alot away to friends and neighbors.