Hello Laura,
I'd like to add my input having experience with the Seneca Casinos. If you are having full tables use the time session fee. $7 or $8 per half hour is appropriate. If the number of players is only six or seven handed I would consider a percentage instead. The nice thing about collecting time is: the dealers take time on their first hand, so you don't have to worry about counting the pot. You must have good dealers though because players will complain about not seeing enough hands if the dealers are slow.
Omaha takes longer to deal, so collecting time might be the way to go.
Another option you might consider, although I haven't seen it in a long time, if the limits are real high (as yours are), you can take the rake from a "time pot."
Players find it less painfull if "time" is taken from the ante's and blinds from a specified pot.