Using the tach is fine, but you''ll get a better setting using a vacumn guage. Vacumn guages are not very expensive and most auto parts store have them. With the engine not running hook it up to manifold vacumn, the tee where the power brake vacumn line is attached, and adjust for the highest reading. You should be able to get some where around 15 to 20 in of vacumn. When you get the highest reading back it down about 2 ins.