With one hand, press lightly and evenly on the clutch pack to be sure it's seated. Using a set of calipers, measure from one of the friction pads of the last fiber you install up to the top of the black outer clutch basket. Then measure the step on the hat, this is the amount that the pressure plate protrudes from the hat on the bottom side. Subtract the two numbers and that will give you your current air gap.

For accuracy you can swap the last two plates and put the steel on top to take your measurements.

Warning: If you use the above method, you MUST put the two plates back in the correct order before putting the Slider hat back on!!! Failure to do so will result in damage to the Slider pressure plate.