I will take a swing at this.
Assuming your seacock is in the hull
just thread the new thru-hull thru the bottom into the seacock. It will bottom out at some point, stop turning, run-out of room in the seacock. Say, for example, there is now 1" between the hull and mushroom of the thru-hull at that point. He is saying take it out and cut 1-1/4" off the thru-hull. This way it will thread into the seacock and the mushroom head
will snug up to the hull with 1/4" more room to spare in the female threads of the seacock before the thru-hull male threads run out of room in the seacock female threaded area.
I hope this helps!