15mm pinhole and 8x10 image???? Be prepared for a truly hole-y amount of vignetting.

Falloff follows a cos^4 for an ideal pinhole.

For 160 degrees, the half-angle is 80 degrees, cos(80)^4 = 0.0009 = 10 stops down.

But you probably won't be able to get even that unless your pinhole is made in a material that is infinitely thin.