Your numbers come out to almost exactly the formula I use for my stands. I would not change the first stand height of 30" with a tank height of 12", but what is the tank height for the 32" tall stand---? 10" ?
As for stability, if the tanks "footprint" is square, it will be equally tippable from any direction. In this case, the four bottom edges of the stand should extend beyond the stands top edges by 1.25" to 1.50" on each of the four sides. Pick one number and use it on all four sides.
For example, if the top of a square stand is 24 x 24, then the bottom should be either 26.5 x 26.5, or 27 x 27. And you could compromise as 26.75 x 26.75 .
If the tanks "footprint" is rectangular, the longest sides at the bottom of the stand should stick out further than the shortest sides. Make the longest two bottom sides extend beyond the top edges by 1.25" to 1.50" (pick one #). Make the shortest two bottom sides extend beyond the top edges by as low as 1", or they could be equal to the two long sides. The reason the longer sides must stick out further, is because it is easier to topple a rectangular tank if rocking it from front-to-back, than from left-to-right.
For example, if the top of a rectangular stand is 30 long x 18 front-to-back, the bottom should be 32.50 long x 20 front-to-back, or 33 long x 21, or a compromise of 32.50 long x 20.50 front-to-back.