Activity in IC/CR

If you are measuring a rimmed tank (Aqueon, Marineland) I wouldn't worry. The trim thickness on those tanks is usually 1/8" more so the focal point of the weight of the tank is mostly on the flat part of the rim, if you look on the bottom rim the outside corner has a round on it from the injection molding and this does nothing for distributing the weight of the tank.

But the other thing to point out is that you have a 4' x 8' sheet of plywood and your tank is 2' x 4'. Instead of cutting off a 24" wide section off one end, just cut the exact size you need from the sheet. You have enough excess material, there is no need to make a center piece like shown in the pic above.

 

Just a hint for you (experience here) the #1 thing to make sure of is that the tank is supported by the corner. All rimmed glass tanks up to 48" long need only technically be supported by the corners, 6" from each corner in each direction to be specific (this comes from the manufacturers).

I built a stand a lot like yours (RC DIY Stand Template thread) and put the tank on it only to find out 3 days after I filled it that the corners were floating. I was so focused on the stand-floor connection (crowned floor and finished basement) that I completely neglected the tank-stand connection. I had to tear the tank down and put plywood & shims on the stand, it sucked.

Also is this for a glass rimmed tank or an acrylic tank? I guess I should have asked that

 

why 2 pieces? one sheet of 3/4" is plenty for a rimmed tank. Technically you don't even need a topper for that stand with a rimmed tank, as long as you have full contact (preferably all the way around, but just in the corners will do)

I personally would not cut multiple pieces. You want one solid piece. Otherwise inconsistencies in the top frame will be translated through the plywood, defeating the purpose of adding the plywood.

Ideally if you have one solid sheet of thick plywood on the top of the stand, when you lay it down on that (and if it is nice and flat) then you can visually see the gaps between the plywood and the frame. Fill these in with shims all around, then screw the plywood down - with the heads slightly counter-sunk, pre-drilled. This eliminates the factor of the screw head pulling the plywood down, but you don't want the head to stick up above the plywood at all once the tank is on it.

the plywood that was the go-to for a long time was Aruco brand out of Brazil, available at Home Depot or Lowes (one of the two, but not both) it was 23/34" and had 4 orange stripes on the end (ID mark) and it was super awesome because the top lamination was thick so you could cut it with a circular saw and it wouldn't chip at all, then the factory burned to the ground and I haven't seen it for a few years.