Leveling a Tank

[gregzz4]

If you're worried about it being closer to perfect, and for your 40b, I use the composite shims from home depot
These may be similar to what I used. Mine are man-made composite plastic and grey-ish
They don't crush like wood and are easy to score and snap

As for your leveling, my 125g was out 3/8" left to right and no worries for over a year
My 75g is perfect using the shims

Just keep in mind that you may want to re-level it after it's full and has settled for a day or 3