ORIGINAL: LBR
A lot of variables to consider: your release, how many strands in the string, nock fit, silencer weight and placement, point weight, etc.
My ball-park guess would be 60-65# spine with the shafts cut 29" BOP with 125 grain points.
Chad
Chad has something on the string. I had a dacron and 60-65 was all stiff. I had Chad make me a 452 string and now everthing showed slightly weak. It made a big difference.