MM (John jumping in the deep end)
I think you are thinking about calculating the weight you hang onto the rod of an assembled crankshaft assy. A method commonly used with Harley's and Big Singles.

To do a twin, where the rods are not on the flywheel, you need to consider the weight of the big end of the connecting rods in your calculation.

So you use 100% of the rotating weight of the connecting rod (Bottom end of the rod and rod bearings), and in this case 70% of the reciprocating (top end of rod, rings, pistons and circlips).