Search for 'felt lap vents' you'll see there is a plastic insert you can get to do exactly this and ensure the gap is maintained. I dare say the same could be achieved with any self-made wedge. Their primary market will be retrofit I should think; from a fresh re-felt being intentional about the sag you leave in you maybe don't even need anything added to maintain the gaps.
In my situation I've found lack of sag in the felt and/or narrow rafter centres hindered me adding as many felt lap vents as I originally intended to (was aiming for whatever the building regs approved doc said the recommended free area was at the time) but anecdotally at least those I have added certainly seem to have increased the overall ventilation sufficiently - I've not witnessed any condensation form on the underside of the felt since, which had been a problem a few winters ago.

Thanks kebl2825 thats perfectly answered my question.