Somewhat flexible? The flange is generally attached to the subfloor and can accommodate various floor thicknesses, within reason.
Warming up the wax ring helps if the flange is well above the finished floor, then a nice firm, even push onto the floor and snug up the floor bolts.
If the flange has never been changed you should be good, usually the floor gets thicker after waves of "re-muddling", then you can add spacers or double up the wax ring, as needed.