Assuming the 2.5" is the finished size ...
You can have the inner border at 1.5" and that would be 21 squares across the short edge and 25 squares down the long edge.
After that, you have your 2.5 blocks as a border.
For the next border ... you can make it any size you want. You are not constricted to matching this next border size to the size of your blocks (unless it is a pieced border). So you can make it 4" ... 8" ... whatever.