On further research it seems that in this case I could have brought the polystyrene all the way down to the roof deck and flashed it with the shingles (cutting them back 2") to make the front of the polystyrene the rain barrier. Assuming I could do a good enough job to guarantee no water penetrated behind it. This would prevent the need for the 2"x1.5" board all together and the gap all the way to the sheathing which would be a big source for thermal bridging and transfer. Not sure I am confident enough to take this approach though and the flashing has already been installed by the roofer.
So my question remains, how big should the gap be above the shingles.