i have a nightforce with a 50mm bell and its mounted in medium rings which are .855 in height and its perfect, for my bolt action. im using nightforce rings and thats the lowest height rings they offer making them their low rings but badger refers that height as mediums
the main problem with the ar is trying to keep the scope as low as possible without sacrificing cheek weld. when mounted too low cheek weld is uncomfortable and you will notice that it forces you to crouch your head over to veiw through the scope.
with the 1/2 height added from the predator rail i would suggest mediums. that would add the .5 to the height of the rings and if you are going with badger that would make it 1.35 which is a hair shorter than what badger recs. for mounting on top of an ar15 which is 1.4
i have used the mediums from badger for a supersniper which also has a 30mm tube but a 42mm bell on top of the arms rail ext and it works fine for me but a hair taller than perfect but for you it should be really close to perfect.
had highs on it at one point in time but it was very uncomfortable with the cheek weld.