If you have an overflow and sump, it's not acceptable unless it tilts toward the end of the overflow. Once running, it will make you crazy unless its level, especially since you tried to get it level to such a degree. Incidently I may be biased as I have a 1,000 gallon/hr return on one thats not level and a ball valve to cut it back to just a couple hundred gallons for just the same amount of unlevel as you have now because the waves go over the side of the tank rather than go over the dam. I wish I had noticed it earlier and fixed it. I thought it wouldn't be a problem but that turned out to be a bad decision. Also, if that stand is not square and level cross ways (X) as well, fix it so it doesn't torque the tank once its full of water so you don't have to worry about it. If it's basically a squared up stand and your overflow is low enough, you may not have the same experience as mine with waves over the lip of the tank. You just have to evaluate that at your end. A 3mm tilt won't break the tank so long as its an even square stand support on all sides but level is always better. 3mm will affect the overflow to the sump if there is one.