I'm replacing a 3/4" tongue and groove rotted bath floor, and want to put in
a tile floor. I've read different things in different books (and on
different web sites) in terms of how to do the floor.

What I need to know is the appropriate thickness of plywood to use (and
grade), and thickness of backerboard. Is 3/4" plywood and 1/4" backerboard
more than I need? The studs are 16" on center.

My goal is to use the thinnest option that will still do a good job (I'd
rather pay a little more for thinner underlayment if needed) to minimize
some potential problems with toilet height, drum trap height, and clearance
on the bottom of the bath door.

Anything will be firmer than the old rotten floor, but I want to make sure
I'm not setting myself up for a lot of grout issues if the floor isn't firm
enough.

Thanks!
Keith