Part of working out the advance curve is limiting the amount of the curve. This is a simple task in a MSD distributor. Underneath the advance assembly plate is a locknut and washer (they're fastened to a post on the bottom side of the distributor shaft). Once it's removed, you'll find a bushing that can be replaced with a new one (larger or smaller). The bushings are shown in the right of this photo. The smallest bushing (red in color) allows for 28 crankshaft degrees of advance. The silver bushing allows for 25 degrees, the blue bushing allows for 21 degrees and the black bushing (the largest overall) allows for 18 degrees of advance.