Try measuring the inside diameter of a fired casing. That will give you an idea of how fat the chamber/throat area is. Then just size your boolits to .001" less than the measurement you get. Example: if the inside diameter of the fired case mouth measures .380" then you should size your boolits to .379".
This method is usually a lot easier than slugging bores and hasn't let me down yet with leverguns.
Hope this helps,