That would work but the backstay is three pieces. It splits into two part way down.
Actually, the numbers sfbay posted are incorrect. Here's the
specs for your Hunter 30, the tall rig numbers are at the bottom of that page.
Never the less, for the lower triangle side, you'd need to figure the distance from mast base to stern , perhaps subtract the "J" (foredeck) dimension from the boat's "LOA". Use the "I" for the other side... then plug in the formula.
or 40(I) squared + [30.4(LOA) - 12.83(J)] squared = backstay squared. Once you have the number for the backstay, you can estimate the split point....say 1/4 of that length, use that number for the split stays' side, and 1/2 the distance between the two stern chain plates for the base. Use the formula again. voila.
However, it might be easier to call a rigging service and have them figure it out for you.