Just a correction;
There are about 9000 players registered but I believe only 40% of them actually plays HP.
Online now; 43
43*48(halfhours/day(because updating every half hour))=2064
9000/100*40=3600
I was almost close. Calcs says 2064 while I say 3600. I have more faith in HP then I should.
How do you know the average amount of people on at any time is 43? Stop making assumptions. You don't know if the time you took that data was the average, the maximum, the minimum, or somewhere inbetween.
It's a decent guess.
However, I'd point out that I'm probably on for 1 hour a day, so with 38 people on as I write, that's about 38*24 people, or 912 people active now.
Suffice to say, there's probably between 1 and 2 thousand people active (defined as on once a week or more), which means that 1.5-3 % use the forums.