How should I shim the feet of a modular tub?
It's a trial and error thing. Place it, feel for wobble. Pull it out and make adjustments. Or avoid the whole fiasco and use mortar, which is the technique the pros use for obvious reasons. It's flexible and can be done in the blind.

And the reason it's necessary is because fiberglass things tend to be less precisely built than, say, wooden furniture. The front panel is left slightly longer to be sure that it rests tightly against the floor.