Incepi fixtures reference the end face of the axle for proper positioning. Spacers are provided for standard or typical sizes (100mm OLD fork 135mm/142mm OLD frame). These spacers are for convenience only and are not necessary for fixture positioning.
With many axle, hub and rotor options, Shimano documents will help you decide which spacing is appropriate. Relevant Shimano documents are included with the purchase of any fixture. Please contact Incepi for clarification or assistance with any fixture offset questions.