depending on the numbers you may get a neg activity level
(5/10)-(20/5)=.5-4=-3.5
i think you would rather make it into an abs
for the second part i'd do (most people+posts made)/people online
it wouldnt be the last hr it could be at the time the forum was the busiest or the total posts on the forum if that was the case it could never hit 100%