I've looked and lookedd but can't find at what height the toilet mounting flange is to be from the finished floor for correct installation of a wax ring and toilet bowl. I have a concrete floor that I plan to put tile on, and the plumber on rough-in left the drain pipe about 3 inches out of the floor. By the time I put the flange on it will be 4 inches high. When I put a 3/4 inch floor on, it will be just a little over 3 inches high. Surely that is way too high. One discussion indicated it should be flush with the floor, but that wasn't the topic of the message, so it wasn't really clear if that was the case. Even my plumbing "how-to" books don't mention this VERY IMPORTANT detail!

Let me set you straight.
Toilet flanges have some flexibility if they're too low but very little if they're too high. The ideal position for a regular closet flange, (see image) is setting directly on top of the finished floor or cement. It isn't necessary to recess them level with the finished floor.
There are flanges that are designed to install when the stubup's located under the floor line but that isn't your problem.
Let's address your installation. I hope the plumber installed a styrofoam closet wrapper on the stub-up before the cement was poured. This is a piece of styrofoam about 3 inches wide and 1 inch thick. This prevents the cement from encasing the stub-up and preventing the flange from seating directly on the finished floor. If not you're going to have to chisel out the floor so the flange can seat level or install a inside flange
The drill wiil go like this. First make sure that there's enough room around the stub-up for the lip of the flange to recess into and tile the floor leaving space for the flange to fit in.
After the floor is tiled you may cut the stub-up level with the tile floor, prime the flange and pipe and glue the flange in place. Good luck, Tom

Thanks Tom for your reply, which I think answers my question. I assume the toilet flange you pictured is upsidedown! The flange I bought has a rubber o-ring to prevent gas flow, and fits down inside the 4" dia. PVC soil pipe, so this styrofoam thing you mentioned doesn't seem relevant. The poured floor comes up flush to the outside of the 4" PVC pipe. So, I need to cut off the PVC so it is even with the finished floor (when installed), and then, after the floor is installed, insert the flange in the pipe, which should then be flush with the floor except for the lip of the flange itself -- right? Thanks, again.

Now that I think of it, shouldn't I also glue the flange down? Otherwise, what holds it in place?

You are correct. The outside flange pictured is upside down. You have a inside flange with a "O" ring.
"Now that I think of it, shouldn't I also glue the flange down? Otherwise, what holds it in place?" The "O" ring seals the but doesn't secure it. Since the "O" ring is the only part that touches the pipe the flange can't be glued and will have to be screwed to the floor.
You should have got a instruction sheet with your flange.
Some flanges have tabs that lock them into place but if your doesn't you will have to set the flange and screw the flange down with Tapcons or drill 1/4" holes in the tile and cement and using plastic inserts screw the flange down that way. Good luck, Tom

Thanks again Tom. I'll drill holes in the concrete and screw the flange down. BTW there were no instructions with the flange; they just sat loose in a bin at Lowe's.

I'm also having to deal with another situation. That is, installing drain assemblies that lead from the tail pipes in several vanities to their respective 1.5" PVC drains in the wall. Every situation seems to be different, and none of them easy, each generally requiring more than one trip to the plumbing supply shop. You are probably nodding your head about that. If I had a huge supply of 1-1/4 in. straight, els, and js, it would be a lot easier I suppose. And why the heck did somebody decide that kitchen drain systems should be 1-1/2 in. and bath drain systems should be 1-1/4 in. I don't really expect an answer to that - ha!

" And why the heck did somebody decide that kitchen drain systems should be 1-1/2 in. and bath drain systems should be 1-1/4 in.? I don't really expect an answer to that - ha!"
You're going to get a answer any way! The size of the drain determine the fixture unit number given to a certain fixture. The number of fixture units will tell the plumber if he's undersizing the drainage when he lays it out. And now you know the rest of the story. Regards, Tom