Agreed.
At one club I teach they book double slots for ALL nav-exs...(based on a 1:30 slot - although given that slot I would say instructing is above average - not just because I teach there but due to the organisation involved.
At another one...a double slot is still cutting it fine..1:15 slot times - so still only 2:30 - which as you say for a first dual nav is tough going....im trying to get the CFI to see sense (seems to be stuck in 30hr ppl mode...read the tiger moth manual & away you go chap...)..
oh which leads me into another one...find out if the aircraft slots are the same as the instructor slots....good schools will have them different say running a staggered instructor slot system with 3 instructors per 2 aicraft type thing.....
Hope this helps