MATH D0 "SUMR(D1, 7200)"
If you don't want to do that, I'd iterate through using a loop and indirect addressing, so the MATH expression will be manageable.
At least once you have the sum, it's much quicker to maintain. Every time you index, add the new value and subtract the one that got pushed off. In fact, so long as you start from the beginning, and it's initially filled with zeros, you can do that while you're filling it too.