IMHO. You should cut it slightly short of bottoming out inside the seacock.
A half inch or so of thread engagement is all that is required in pipe
fitting.

When you tighten the valve onto the thruhull, don't make the common mistake
of over tightening. This would squeeze out all of the caulking compound.
You have the correct idea in leaving a think film of sealant between the
thruhull, valve flange and hull.. (in installing deck hardware, etc. I bed
them by only lightly tighening the bolts, I then come back after the sealant
has cured some, hold the bolt and tighten the nut until everything feels
firm. Don't let the bolt turn.)


--
My opinion and experience. FWIW

Steve
s/v Good Intentions