what is the highest drop i can take on my giant iguana with the mx comp fork on it?

Not to sure on the answer, to many variables to think of.
lets see....
-first: what your landing on, a flatland will be alot harder on the bike, and a landing on a downward slope or a transition on a jump wouldn't be as hard.
-second: your skill of riding, better skill and you can pump with your body and take part of the drop using your body as a kind of suspension.
-third: the BIGGEST thing is just how *%$#@ed you are in the head and how willing you are to just throw yourself off a huge kicker, gnarly road gap, or best friends porch.

I'm no scientist so I'll leave the math to you WNCbiker....
LATER

I agree. It depends on landing. Tranny, rider weight. front heavy rear heavy. The frame should hold up to quite a bit (I wouldn't go over 6 ft but I am 250pnd) but the rims may break under that kind of duress. Especially if you have stock rims which are crap. (a friend of mine did a 7ft drop to flat and broke the stock rim in two. He is a small rider and smooth).

Anyways, too many variables to help you out much more than that. Do a 3 footer. If you hear creaking or cracking and the rear rim is slightly off you found a limit to your skill and current bike setup

i have a 01 norco rampage and have done 8 footers to trannys and the landings were pretty smooth. However that is after i put the Intense mag 30 in the rear ( I busted my ryhno lite rim in half, It might have been from the 5 -6 footers to flat that had something to do with that.)
I agree with them start off small and listen to your bike. U never know how big u can go until you've gone to far.

4 to 5 feet.