Yes, that is indeed the formula, if you are currently running Bravo 1 props. Bravo 1 props are one inch less than is stamped, on the propeller.
When you add a blade you will reduce one inch of pitch to compensate for the increase drag of the additional blade.
So... if you are running Bravo I props and want to add a blade you will reduce 2 inches of pitch.
If you are running propellers other than Bravo 1 prop then simply reduce one inch of pitch when adding a blade.
Julie