SAT (Standard Aviation Thumb) when I was taught in the RN was always 10nm (for a half mil chart), mine is slightly short at 9 so I would have thought 7 a bit on the short side - might also be what point of your thumb you are measuring to -as long as you know what distance your own thumb is that is the main thing, though 9-10nm is good as that is near enough 6 mins still air in many training aircraft.
