If possible, I think it would be good to be able to enter equations into grid cells.  Instead of applying to all values for a given field in the database, the equation would apply on a per-item basis - only to those cells to which the equation has been entered or copied.  It would work very much like entering formulas into spreadsheet cells, easily copied and pasted, and would therefore be something everybody is familiar with.  More intuitive than dealing with equations in the Manage Fields dialog.