I've tried to pay attention to pro players while they are seated at the drums (dums upright, head parallel to floor). In particular, when they are resting their hands on the head (as in during explanations on instructional videos). It seems to vary a small amount, but with the palms flat on the head, the typical position seems to be close to 90 deg, or slightly less (the hands slightly higher).
Players I've noted include Patato, Candido, and Giovanni (all with drums standing on end), and Bobby Sanabria with drum held between the legs and tipped away. Giovanni seems to play with the drum head a wee bit higher than others--but then, if you want to play with your elbows you're going to have to, I think

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Kalani also corroborates--"about 90 degrees".