ignition timing ?

I can't comment on the accuracy of the number you used and the amount of advance it would give you but the one time I modified a stock distributor I didn't bother with a measurement.

I removed the point/pickup plate, then glued a 39 cent protractor to an old rotor so the center of the protractor was centered over the center of distributor shaft hole. I put the rotor/protractor combo on the distributor with a wire taped to the housing to use as a pointer and advanced the distributor to 1/2 the number of degrees I wanted on the crank (distributor degrees are 1/2 crank because the distributor rotates at 1/2 speed) and skribbed a line. I then welded the slots to that point and fine tuned it by measuring with the protractor to where I wanted it with a file.

When I put the distributor back in the engine and measured the timing on the timing tape it was dead on.