A level tank is a HUGE deal. A tipsy tank - esp over 100g is a disaster waiting to happen! Remember the total weight of your system will be ~ 3000 pounds and glass tanks can shatter a wall or bottom if not level AND flat.
I'd say the tank needs to be level within .2" in either the long or wide dimension. My 475 was off by less than .25 lengthwise - and I was worried about it so I placed cardboard boxes (flatened out) under the tank starting at 1/3 the way down the length. the 2nd layer of them was placed at the ~5/8 point, and the 3rd shim was placed at the 3/4 point. The 3 cardboard shims brought the low end up by ~ 1/4 inch and when filled - the tank just barely setteled (compressed) the cardboard and the tank is now level within .1" lengthwise. I'd shoot for 1/8" MAX level in both dimensions on yours, for sure.
I'd NOT use cardboard as a shim for any raising of more than .25" though. I'd suggest using real thin wood (like paneling) to build a stepped platform to raise the low end of the stand. It would likely take probably close to 16 different length sheets (if each one is 1/8" thick), each one about 6 inches shorter than the next - to raise the low end ~2.0" .. I'd also start the shims at about 6 inches from the high end and evenly space them 6" to the nest and so on... and build up a level shim stack - and never have more than a 1/8" gap anywhere under the stand. I'd also shim the stand @ the floor - not just the tank.
Or option 2 - have someone build a custom stand for the tank that compensates for the unlevel floor - but it'd probably run a few hundred just in materials - and it'd have to built very well, or the tank could burst/leak. Using a custom stand will TOTALLY void any warranty on the tank though.
If it'd go on the other wall where it's only 1/2" off - I'd still level it, using thin wood sheets, but only ~4 full tank width (96") would be needed. Again i'd shim the stand/floor, and i'd evenly shim it from the high corner.
Keep us posted - but the work you do now to get it as close to perfect will totally pay off later with piece of mind that it's good to go, and the chances of a catastrophic failure are minimum. Who wants 200g saltwater all over the house?
my plan now is to move everything from both my 90g tanks into the 240...one tank is semi reef, 2 clowns, 1 blenny, 1 pseudochromis, cb shrimp, cleaner shrimp, hermits, nem, zoas, shrooms, leathers, xenia...the other tank has 1 yellow tang, 1 niger trigger, 3 damsels. both tanks have 100+ lbs of live rock and 50-60 (?) lbs live sand....how long do you think it would take for my trigger to hunt down my shrimp and crabs (he's a small trigger 4-5 in) and make dinner of them?
