I am putting a bathroom in my basement, and about to install the flange. From reading posts here, I saw 10, 12, or 14 inch rough ins. What is the most used distance, and then, is it measured from the finished wall or the finished studs? Also, since I will be putting tile down, should I install the flange on top of the backer board?
It depends on the toilet that you plan to install, but the most used distance is 12" on center from the finished wall.
For the best wax ring seal results, the toilet flange should be installed on top of the finished tile floor level with only the thickness of the flange itself above finished tile floor level, and bolted securely to it with the upright toilet bolts at 9 o'clock and 3 o'clock.
If you're bolting the flange down to wood, then lag screws work well. If you're bolting it down to concrete, then Tapcons work well.