On this boat there is no backstay, you’re looking at the topping lift running back to the boom.
Try releasing any tension on the mainsheet. It may not do much, but it is simple to do. Generally is best to tune the mast before installing the boom.
I have a loos guage and used it to set equal tension on the upper and lower shrouds. That said I’m going by articles online and YouTube videos so I will see if I can find someone who knows what they’re doing.
Ah, the internet, a vast collection of unvetted information.
Tension is usually set based on the wire size. I use about 10-12% of breaking strength as a guide. Start with the all rigging just hand tight and make sure the mast head is centered over the boat. l use the outer edge of my chain plates, others are more fussy and measure a points on the rail that are equidistant from the stem. Adjust the tension by hand on the uppers to center the mast. Leave the lowers loose. Now gradually bring up the tension on the uppers, go back and forth a couple of turns on the port side, then a couple of turns on the starboard, check the tension and check the measurements. Keep repeating until the desired tension is reached. Check the mast for straightness by looking up the mainsail track. If everything is good, then move on to the lowers. If not, loosen it all up and shake the mast a bit to remove all the tension and try again.
Next move on to the lowers. Start with hand tight to an even tension on both sides. Check for mast straightness along the mainsail track. Repeat the tightening process from side to side, checking mast straightness and tension until the desired tension his reached. When I tune my rig, the lowers have a little less tension because they use smaller diameter wire. If your wires are the same size, you might try a little less tension on the lowers, so if you are 12% on the uppers, try 9 ir 10% on the lowers.
Now tighten the forestay. Finding the correct tension will be some trial and error. It should be tight enough that the roller furler works easily but loose enough that there is some sag in the forestay when going to weather. Make your best guess and go sailing.
Check the tune by going to weather in 8 to 10 knots of wind. The leeward shrouds should be just slack, not slopping around, just a little softer than when you tuned it. If they are tight back off a turn, if too loose, tighten up a turn or two. Tack and repeat for the other side. Check the mast for straightness.
The forestay should sag a little, maybe 2 or 3 inches to match the curve of the luff and the sail should furl easily.
Back at the dock, check the tension again to see that the pairs of shrouds are about equally tensioned.
The best places for reliable information on rig tuning will be from spar manufacturers, big name sailmakers, (North, Doyle, etc), and one design class websites. The most specific information will come from a one design website for your specific boat. The information I provided is based on experience and reading books, yea the paper kind that an editor actually vetted for accuracy.
Ivar Dedekam's book (Amazon link) is one of the better more comprehensive books available.
Good luck!
