Ah, right, he has that extraneous cylinder. The equation is fine if only he would install his rods first which, I realize, no one does when balancing a twin, if for no other reason than the friction of the plain bearings in the big ends. Of course, when balancing a twin the flywheels are much further apart than on a single so the potential of a big rocking couple is there which is why dynamic balancing should be used (I know you know this, John, but it's worth mentioning in case the OP doesn't).
Of course it has nothing to do with having 2 cylinders or just 1, but whether the rods are easily removable, ie. have end caps for plain bearings. The same procedure would be used for a Starfire single (plus you wouldn't be risking wearing the bigend by hanging a bobweight off its rod - joking!).
One thing the OP mentioned was "I make balance weights tape them on the crank".
If they are taped to the crank, isn't it important that the weight is evenly distributed around the bigend journal? Or better still, hung from "frictionless" loops around the journals? So that the force acts from the correct distance from the crank axis?
I thoroughly endorse MM's recommendation for dynamic balancing in the case of a twin. Mine cost £70 20 years ago at Basset Down, it takes care of balance factor and the surpisingly significant rocking couple at the same time. If it's twice the price now I'd say it's money well invested.
I've done one rebore since, and all I did was minor scraping inside one piston to get them to match, not concerned that they were a few grammes heavier than the old ones, still a lovely smooth motor.
Tracey, a good calibrated (use standard weights or coins) set of scales, a horizontal surface, 2 small round topped things that will allow free movement of the conrod placed upon them (I find the handles of 1/2 round needle files good for this, as the 1/2 round keeps them from rolling about), and a block about the same height as the scale pan. Set the 2 round things apart by the distance of the big and small end centres. Zero the scales, place the rod so the bearing centres are on the round things, note weight. Turn rod around, note weight. Put rod on scales, note weight. If you've done it right, the 2 smaller weights will sum to the total.