I wouldn't recommend machining down the original at all, especially if your going to be putting in a heavy clutch. Can be very dangerous, much better off getting a off the shelf or custom made light flywheel as they're engineered better.
A guy I know in Perth had a machined stock flywheel and an extreme puck clutch in his VL (yes holden POS i know)...but one day whilst doing some 'spirited' driving it let go and blew apart the clutch/pressure plate/bellhousing and he was VERY lucky not to have got a piece of it through the firewall and into his leg!
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