In my experience you won't get a dead-solid mount on a hard floor about 90% of the time. The toilet base and the tile are too likely to have imperfections that affect the connection.
So, rely on caulk. Clear, 100% pure silicone in a bead pressed into the void under the toilet will make it solid. I usually do the front and sides, but leave the back open. It's not necessary to do the back and it's a good place to leave a leak indication drain.
If you don't trust your caulking skills, mask the floor directly under the outer edge of the toilet base (or slightly inside). Inject the caulk in small beads and work it into a cove with your finger. Add more only if it presses in too far.
Give it a few hours to fully cure and be happy.