[kjflyfish]

Assuming 110 cm of band stretch and a 7cm wishbone:

110 x 2 = 220

220 - 7 = 213

213 / 3.5 = 60.86

So the optimum (circular) band size for a gun with 110 cm of band stretch is 60 or 61 cm, or 30 cm for screw-in bands.