The best way to tension your stays is with a tension gauge. You need to know the thickness of your stays. Then, going by chart on the gauge that corresponds to the thickness of your stays, you need to pick a number out of a range of three or four numbers for your outer stays, plus the fore stay and back stay. Then you should tension your inner stays a little less. I use a Loos Gauge for my stays. They're kind of pricey but they do the job. The good thing about using a tension gauge is; if you have the same tension numbers, it stands to reason that your mast is going to be straight. I would pick a number on the gauge which is the lowest tension in the range for the thickness of your stays and go with that.
Joe