Well . . . .
Assuming that your rig is standing and has a degree of tension, I would start with the shrouds first, uppers and then lowers working one side of the uppers, balance with the other side, then the lowers. Perhaps saying the obvious is to insure that your mast is a straight line and not bent or bowed to one side or the other. Some masts will be bowed to the back like the H340. But latteral bowing/bending is not acceptable. Such bends/bows not only affects performance, but makes it affects the strength of the mast and contributes to resistence when raising the sail.I'm not sure what your rigging looks like on the 280. I assume that you have a backstay since this was produced in the 1980s. Whether you tighten the forestay or the backstay first depends on if you want your mast vertical or raked back slightly. If you have additional lower shrouds that angle forward and backward from the main shroud, as Catalinas and Beneteaus do, you'll need to adjust those after you have the forestay and backstay adjusted to your liking.I rarely get mine mast tuned in one setting. A few good sails tends to make a second adjustment necessary to get the mast to where I would like it. And being a little fanatical, I try to tune it at least once a year.If you have no backstay, as is my situation on the 340 (shrouds 120 degrees from forestay), the adjustments take on a little different sequence. But I don't think that is your situation.BTW, if you do not have the Loos pamphlet, you could give me your wire cable sizes and I can give you the % breakage for each Loos reading. Or worse come to worse, I could email you a copy.A well tuned mast really helps performance as well as reducing wear and tear on the boat, and may help reduce weird vibrations in higher winds.Please feel free to ask more questions.Good luck.