I use an automotive style vacuum/pressure gauge that goes to 15 psi...at speeds above idle it already reads 10 psi and over 15 psi at WOT. Idle pressure is 5 psi, this is on a 90 hp Merc two stroke OB.
If I was going to do it again I would get a marine gauge that reads 0-30 psi, this model is pretty nice and comes with all the hardware plus is backlit.