On my Mk2a, 12 or 16 was a good setting for ReserveCache when running just emphatic.

Thing is, I don't think you're going to have success running gpsapp and emphatic at the same time. The problem is, two apps tend to fight with each other in terms of controlling the display, etc. emphatic has a mechanism for going "to the background" with the "Hide" menu option, but I remember people complaining that the two still wouldn't coexist. As a non-gpsapp user, I really can't comment on why that might be.