It depends on whether you have a 180 or 360 degree engine.
A 360 has the pistons going up & down together, and you treat it like a
single, only add up both sides' reciprocating weights and make BF around
66% of that if the cylinders are vertical.
If 180, both sides cancel each other so BF isn't so critical.
Actual figure is found by the factory by trial & error, so you're alone
there.
Add a head-steady if it doesn't already have one.

Jordan

wrote:
I need to know the bob weight formula [balance factor] for an
inline twin [both pistons run in parallel].