I figure you should just set it the same as the gold and see how it works. I also have been thinking about running the harder spring as the gold one just seems to change during the longer mains. It often feels like the clutch nut backs out but after measuring it doesn't move at all.

I usually run 9.3 mm from the end of the crank to the top of the clutch nut when the motor is at BDC at the same point. This is including the .5 mm spacer behind the flywheel and motor. So, I reckon a good starting point would be at 9.0 with the silver mugen clutch spring i'm guessing. I'll try it next weekend and let you know.