Backstay is there to remove forestay sag, and..
to induce bend in a fractional rig (it'll also induce bend in a masthead rig, but the amount will be less, depending on how bendy the mast is)...
Since we are talking about a Catalina 22, there is 1 set of uppers, 2 set of lowers, and YES they originally came with no adjustable backstay, and yes we are talking about a masthead rig...
Since the rig is relatively light, there can be significant amount of bend from the backstay adjustment (some argue masthead rigs don't do bend, they ONLY remove forestay, that's a bit of hocus pocus, it actually does both, just NOT as much bend as a fractional). All "initial" settings should be with backstay completely off.
Set maximum forestay length first and fix the pin (measured pin to pin), see class association rules, this setting might leave you with a bit of weatherhelm, but it'll make you a pointing mad genius on the race course.
Set straight mast next... centered port/starboard...
Set proper uppers to class recomended tension, use loos gauge, set for light-med air (or whatever your prevailing winds are.
Set mast rake, and I bet that is probably like stated above 3-4 inches (measure by measuring the distance a weight swings by gravity alone away from the mast from the main halyard, measure this distance at gooseneck height).
Tighten forward lowers to hand tight (leave aft ones loose right now), check mast straight quickly again.
Now tighten forward lowers until you induce a slight "prebend" usually 1/2" to 1" check this by resting the main halyard with tension against the mast... the amount it "diverts" at the spreaders is prebend...
Take up the slack in the aft lowers by tightening to hand tighten again.. add 1 full turn past hand tight evenly both sides.
NOW, measure your rake again (should be same as before might be slightly more with prebend, no worries)... now go crank on the backstay to max... measure your rake again... you will likely see an additional 3-4 inches of mast head movement (cant' really call it rake because the mast has bent some).
From there you sail... its all experimentation with YOUR sails, how the tune will work. if you go sailing, get on a nice beat in 8-12 knots wind... get at about 15 degree of heel on a close reach... check your leeward shrouds... are they flopping? If so take up the tension until hand tight only, counting turns to get you there. make note, so you can tack and add the same amount of tension to the other side.
trimming the backstay will require you to watch your headstay sag while sailing. If your winds start to come up, look at the jib luff, and curve of the stay... generally if you raised your genoa, and there were no "scallops" or only light scallops, and the winds picked up significantly, then the scallops will become more pronounced. This means you have more sag. crank on the backstay to see if they go away. if you can get your shape back, great, if not, you may ALSO need more halyard tension (winds REALLY picked up)... Ease as winds drop off... if your halyard is a banjo string, then ease a bit. Generally you want to have halyard with just a slight bit of scallops forming in the luff (very hard to see with a furler or a foil).
It's all a game of playing "what is the best entry angle." What gives the best performance, and you'll have to play it a bit in steady winds to develop tensions, and settings. I'd strongly recommend you mark your halyards for positions... I add 3 marks (usually 1" increments) tighter than my lightest air settings... If my lightest air settings aren't loose enough I add the other direction... so that I have a reference for trim settings for each wind velocity. I do the same for my outhaul, and genoa sheets (sharpies work great). I do the rest by feel... I've added P-touch labels too on the boom 1" 2" 3" etc for outhaul... and labeled the exit paths for halyards, top lift, cunningham, for easy identification by crew... when I say crazy stuff like "blow the spin halyard!"
One last tip... add contrasting stripes to your spreaders... if your class recommended max trim position is "2" off the spreaders for the 155, then put your contrasting stripe 2" IN from the tip of the spreader. Its a quick glance way to measure the distance for your trimmer (or yourself) to adjust for optimum trim upwind.
