Are you deleting Document from within a Project form (table of items?) or separately
IF from within a Project form You could try starting a process when the save button is clicked ("Buttons")
which will recalculate the referenced items on the fly
something like SUM DocumentDivision.attribute WHERE (DocumentDivision IN Project.DocumentDivision)
or possibly a rule in Projects like
IF Projects.DocumentDivision WAS CHANGED then ......
Difficult to advise without more detail