Looking for an expert on engines for a question on engine ..

I was directed here by someone on
alt.binairies.pictures.motorcycles.sportbikes

Guys, anyone know of or know where to get information on engine balance?

I have to change the balance factor of the motor I will be using on a café
racer I am building. I found a place that will do it but they are car guys
and they don't
know the specifications for an old Norton. So far I have found that the
proper balance factor for an engine rigidly mounted to the frame should be
84%. What does that refer to? 84% of what? The engine repair shop further
confused me by asking what were the reciprocating and rotating factors????

Just for information, the Norton is a vertical parallel twin. This means
both pistons rise and fall in unison.

Any help appreciated

Jean