All upright combinations = ((M * N) * (M + 1) * (N + 1))/4
Total Diagonal Squares = (M * (N - 1)) + (N * (M - 1))
Diagonal M = Int(Total Diagonal Squares / Upright(N))
Diagonal N = Upright(N)
Leftovers = Total Diagonal Squares - (Diagonal M * Diagonal N)

As long as Diagonal M and Diagonal N are greater than 1, use the following:
All diagonal combinations = (((M * N) * (M + 1) * (N + 1))/4) + Leftovers
Otherwise, All diagonal combinations = Diagonal M * Diagonal N

For Example: 3X2 Grid
Upright Combinations = ((3 * 2) * (3 + 1) * (2 + 1))/4 = (6 * 4 * 3)/4 = 6 * 3 = 18
Total Diagonal Squares = (3 * (2 -1)) + (2 * (3 - 1)) = (3 * 1) + (2 * 2) = 3 + 4 = 7
Diagonal M = Int(7 / 2) = Int(3.5) = 3
Diagonal N = 2
Leftovers = 7 - (3 * 2) = 7 - 6 = 1
Since we know that a 3X2 grid has 18 combinations, for the diagonal we add the Leftovers, which is 1, to 18 and that gives us 19 for all diagonal combinations.

Now rinse and repeat for all smaller grids.