if the chippies done his job right there should be some timber vertical and horizontal top and bottom of the velux 25mm behind that little groove thats designed to take 1/2" board either square to the window or plumb an level (bottom edge/top edge)... wants 25mm of cellotex then screw your board on with 50mm drywalls..
and if not, bang the noggin(s) out and reposition em..
should have 80mm cellotex between the rafters too..
as for dabbin over board though.. great way of packing out silly distance, works absolutely fine, uses up loads of offcuts too..
you can do the same thing with tiles too so your adhesive aint on too thick (premixed adhesive just wont go off over a certain thickness).. think kitchen window cills and the like...