There are shims sold specifically for leveling toilets. Even Ace
Hardware has 'em:
I'd use those to level the bowl, then push something like tile grout or
"Rockite" cement into any gaps between the base and the floor.
Probably best to locate where the shims are needed and maybe sticking
them to the floor with some adhesive before finally placing in the wax
ring and lowering the bowl into place.
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