Tightening bolts on the base of a toilet is the one thing about toilet installation that has me the most nervous. They say not to over-tighten, but how tight is too tight before the porcelain is subject to cracking? Do I tighten until I can't tighten anymore? Do I tighten it just before I think it's at its tightest?
I start by hand tightening as much as I can, then I tighten a little on each side and check if the toilet moves. If it does then I tighten a bit more, check and repeat until there's no movement. You're trying to avoid bowl movement, so that it doesn't shift or fall over, not to hold the floor up by the toilet bolts.
As tight as you can go will probably snap the porcelain (with a good socket you can torque a nut onto a bold quite a bit). Until you think it's about to snap is just guessing.
Snug. No movement. Enjoy.
"Too tight" is a sixteenth of a turn past where the porcelain snaps :-)
One thing that will help prevent the porcelain cracking is to use a rubber or plastic washer between the head of the bolt and the bowl.
This will take the "excess" force by deforming slightly, and by checking for the washer deforming as you tighten you'll have an extra visual check that you've applied enough force.