Don't answer if you don't feel like it, but why are you concerned with nominal gauges? For an 18x20 pattern, use the 17 Ga. For a 16x19 pattern, use the 16 and for wider string spacings, use the 15 or 15L. You should also be aware that when the strings are put under tension, they will stretch and their diameters will decrease. Since these are multi, they will stretch a lot; probably more than 5% depending on tension. 2 cents.