How much "flat" is a minimum for a TT rear end ring gear to be usable?

I don't know what you mean by "flat", but I have always thought that they're pretty far gone if, when looking at the edge of the gears, they're worn to the extent the edges are sharp.

Just my $0.02 worth. Other opinions may vary.

So instead of sharp, there would be some flat surface that wore away until it was sharp.

So how much flat surface at the top of each tooth is the minimum acceptable if sharp is too far gone?

The "flat" is different between high and low speed gears. Which one are you using?

If you slide your finger along the bronze tooth and wind up bleeding, you need to look for a better one.

Tom, I'm pretty sure that during assembly you can turn the bronze gear around so that the worm will be pulling on the fresh side of the teeth as you drive forward. 'Course, if they're worn down to knife edge you should probably replace it.

Here is a post with some information and photos. I was mistaken that the low speed gear was my NOS one from John Danuser. That one has even wider "toes" or flats on it.

http://www.mtfca.com/discus/messages/506218/519532.html?1424348101