I'm no expert, just tiled my first little Bathroom and it has 5/8" plywood.
My main concern was to screw down this existing plywood good, about every 6".
I was told not to use anything else on top, just Ditra.
You have 1/2 "...a little extra wouldn't hurt, if you do, I would screw down first the main floor, then
use Construction adhesive and use a ton of 3/4" dry wall screws.
Konrad

I'd head over to John Bridge (or get Bill Vincent's attention here). The key issue is whether your joists are stiff enough for the kind of tile you want to use. You can use Deflect-o at John Bridge to figure that out. Adding plywood will help, but it won't make up for insufficient joists.

A couple other notes- IIRC, you don't want to glue down the new layer of plywood. I think you want it to be able to move laterally (even if only a tiny amount) independently from the bottom layer. Again, I'm going on fading recollection, but I also recall shooting for the new layer being screwed only into the bottom layer (not the joists)

And don't use drywall screws. They're not strong enough for the application. Get some deck screws, at the minimum.

Bottom line is that you should consult the experts. I'm not one.

Besides joist deflection, the deflection of the subfloor between joists in the lateral direction is also a consideration; make sure the subfloor is more than adequate for the job.

If you're just using the 1/4" plywood as a dimensional filler, I'd use 1/4" backerboard instead and thinset and screw it down. That way the voids are filled underneath and there won't be any possible gaps to give and cause the tile to crack.

If you need the plywood for some added reinforcement, I'd do a full glue and screw with no gaps in the glue. General construction adhesive is good, polyurethane construction adhesive is even better.

I thought about 3/8" to 1/2"..but the problem I have is that my Toilet flange sits at about 1/4"-3/8" above the sub-floor.

When I ripped out the old tile, I removed the old laminate (which I THINK was 1/4" or 3/8" OSB..not sure exactly what size).

I was thinking 1/4" plywood + 1/8" Ditra..and then 3/8"-1/2" for the tile..that is why I was thinking 1/4" plywood...I worry about having the flange be to far under the tile to not allow it to seat properly.

For the beam deflection, not too worried, already did a beam calc, and it came in fine..I just thought te 1/4" plywood would "stiffen" the floor a little more..at this point, I don't fill any bounce given the placement of the bathroom (2 sides are outer walls..so the load transfers down them).

Can't you just get a flange extender?

Do I need to put a gasket between the flange and the extender?

Also, what's the limit? 1/4", 3/8". etc?

What's the most gap between the top of the tile, and the top of the flange I can have before I need to be worried that the toilet won't seat properly?

>>And don't use drywall screws. They're not strong enough for the application. Get some deck screws, at the minimum.Drywall screws will work 100%, especially when you glue another thin piece on top, the glue is giving you the holding power.
When you screw a thin piece on top, dry wall screws have threads all the way to the head, it will still pull the two pieces together, I like them allot better in this sort of application when short screws are needed like here!.....actually they have better grip then deck screws and cheaper.

>>What's the most gap between the top of the tile, and the top of the flange I can have before I need to be worried that the toilet won't seat properly?I think your system is the same there then it is here in Canada,?... are you using the wax gasket?
If so.....
Because I asked this same question to the local hardware store and the guy stated, you can put two
wax gasket on top of each other.
Perhaps ask the same question in your local store.
Konrad