You know the span of the originals. Use a ruler and a triangle and with the triangle set to the same 1/2 span on the ruler as the stock stabs lift the ruller through a 35 degree angle from horizontal and see what measurement the ruler is at where it meets the triangle. That's about how much more you need.

This is very rough and the glider guys would tar and feather me for it but in this case where the ST is such a conservative design this will be close enough.