I’m wondering the best way to allocate expenses to grants on a monthly basis.

We’re a mid-sized NPO ($25m) and have about 200 grants, restricted on department level. We have a software in place, but we don’t code expenses to grants, only to departments.

I would like to run a month end allocation to assign all expenses to the appropriate grants so we can easily run accurate funder reports.

What is the logic that should be applied here? The grants are obviously on different timelines, so is it by % of revenue assigned to that dept for grants covering that month? Can it be based on budget proposal? (This can of course change and make the allocation inaccurate quickly).

Any direction would be appreciated! Thanks!