Alan,
There are basically two schools of thought, for single spreader rigs particularly.
1. Use Loos gauge and measure
2. Use Catalinas extremely good rigging setup guide included in all Catalina manuals. RTFM (F stands for funny, I have to say this or else they'd ban me again!)
For the past 30 years, on our C22, then C25 and our current C34, I have never had the need to buy or borrow a Loos gauge, because the rigging suggestions are sooooo very simple, clear and useful that there is really nothing else that's necessary.
You could also Google "Selden mast tuning" and find some good input.
KISS it's not that hard, even with a two spreader rig.