I'm making a boot over winter and want to make ure that I maximise the space as much as possible which means putting the boot floor as low as possible and therefore close above to the rear axle.

I know there will be no hard and fast rule to this but how much will my axle move up when driving? 5cm? 10cm? More?

Sit on the back and push down hard. You should get a fairly decent idea how much it moves.

It really depends how hard your springs are

It can't move more than the extension of your dampers. Normally 100mm total travel, so 50mm bump and 50mm rebound.

I'd think 75mm fromt he axle when at rest would give you 20-25mm of clearance.

Regards Dan.