I am replacing a thruhull that was coupled to a properly backed,
bedded and bolted flanged base seacock.

Question: Do I cut the thruhull exactly to length plus 1/16" so that
it butts to the inside shoulder and leaves a room for a thin pillow of
caulk against the hull?

or Do I cut the thruhull shy and tighten it down just enough to leave
a thin pillow of caulk against the hull?