You usually bolt the flywheel to a balanced crank, balance the flywheel first (by drilling holes), then bolt the backplate on with the straps and balance that.
Then bolt the diaphragm on as well and check balance again. Try all 3 possible positions and choose the least out-of-balanced one.
If you are really anal you can perhaps drill a few small holes in, but it's so light it won't be far out anyway.
I did one recently where someone had bolted the backplate on and drilled a dirty big hole 24mm in it alone, to balance it AND the flywheel. Almost drilled right through it. I binned that backplate, I've seen what can happen.. they are grey iron and not real `bendable'.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
