Toto Carusoe tank leak

My toto is leaking at the same place - down the big threads of the flush valve base.
Is the thin rubber gasket and the big nut on the valve base supposed to make it watertight alone? If so, just snug the nut? Or would plumbers putty help?

Or, does the tank-to-bowl donut really seal the threads?

NEVER use plumbers putty anywhere on a toilet tank.

We carry large pliers to will further tighten the large nut for the 3" flush valve.

Installing a toilet

Thanks I checked the porcelain and reinstalled, but there is still a very slight drip down the threads when I set it out to test it. This is a fluidmaster 2" valve and gasket on a two-year old Toto Carusoe. The porcelain is clean, no crackis.

Is there a better gasket to try?

The Fluidmaster kit is a tough one to seal.
It has to be tightened real evenly.