For my 350g I marked out where the 8 legs of my stand would be on the floor using dry erase markers. (Tile floor) I then used my 6' level to determine which leg would sit on the highest spot. This then became my benchmark. From this point I leveled to each of the other layed out legs using various size pieces of flat wood 1/64", 1/32", 1/16" and 1/8". I stacked the shims at each of these points as needed to bring them level with my high point.
I then measured each stack of shims and cut pieces of hardwood to that thickness and the same size as the foot of my legs (3 1/2" x 3 1/2") being carefull not to mix them up. I then pin nailed them to the bottom of the legs. It helps to draw the leg locations out on a sheet of paper with the appropriete shim measurements for each leg. Also mark each shim thickness on the shim itself.
After I set the tank in place no matter where I put the level it was dog nuts. :thumb:
Basic answer (without all the details) is that instead of shimming @ 4 corners only you'll need to shim at regular intervals along the solid bottom. Conservative=every 6"/more normal=every 12" ...........
Think of it as a very long, thin wedge. Lets say over the length of 3' you need to have 3/8" thickness on one end and then have it taper down to 1/8" at the other. Start with a striaght pc of 2 x 4 and over the length of 3' mark a straight edge line from the 3/8ths mark to the 1/8" mark. Thier are jigs you can use to make really nice long angle cuts but I prefer freehanding the cut on my tablesaw. Basically it's a really long thin wedge that runs the entire length of the area to be leveled.
Edit: For anybody that doesn't have a lot of experiance useing a table saw I don't recommend free handing open field cuts. It can get away from you real easy if you don't how the blade reacts in that situation and and feeding the wood is different then if you were doing standard rips against a fence.
You can buy packages of wooden wedges known as "shims" from your local home center. Sometimes they are located in the lumber area and sometimes in the door area (since they are also commonly used to precisely level and plumb doors and windows in their openings).
I would set the stand in its intended location and tap the shims into place under the lowest spot in the floor to level the stand. The fill in all the other gaps under the stand with more shims (about every 6 inches). Once the stand is level, you can cut and snap off any protruding wood. The trick with the shims is to tap them in slowly with a hammer until they are tight. If you drive one in too far, however, it will lift the stand up and off of the other shims. Just keep checking the shims and checking the level (both side to side and front to back) . Don't cut the excess off of the shims until everything is level and tight.
I did my stand this way. Their was a large gap under one end and you could see the shims when I was done leveling, so I added a base moulding to the stand to cover the gap and give a finished look.
Yeah the 85 i have right now leaning back some right now but i want to make sure my 300 is going to be purfect. It is going to be very hard to hammer a shim in the back of the stand that is against a wall. I was thinking i could life it up but the stand probably weighs a few hundred pounds.