Re: Santana 21
Many sailmakers offer 'tuning guides' for recognized designs.. perhaps there's one available for your boat.
The tool you refer to for measuring and setting rig tension is called, ironically, a "Loos gauge".
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)