A 1 Galon kit (1 gal A + 1 gal B) of 4 lb foam makes 4 cubic feet of foam.
Maybe someone knows the volume for 170 stringer, but if not:
Measure your stringers (Width at top + Width at bottom) divided by 2 = average width. Estimate average height. Average width x average height x length = volume. If you measured in feet, that's how much foam you need per stringer. If you measured in inches, Divide by 1728 to get volume in cubic feet per stringer.