leafsta

Currently building an extension on my house as a permitted development but i'm a bit out of touch with roofing membranes and the amount of airgap required when insulating between and below the rafters.. from my understanding it's a minimum of 50mm above the insulation so i'm guessing that means on a 175x50 roof, I can use no more than 125 PIR between the rafters... can anyone confirm if thats correct? Thanks in advance!