Updating an inception-to-date formula
 

Does anyone know an easy way to automatically update the fixed value in an
ITD formula?

Let's say I'm putting the current month's value in A1, with an ITD formula
in B1. $5 has been spent to date, $2 more is spent in the current period.

Current formula in B1: =A1+5 (value $7)

To prepare for the next month, any ideas on how to get the formula in B1 to
automatically update to =A1+7 before I blank out the content of A1?

I'm currently doing this with helper columns to write the updated formulas
for hundreds of rows, but that has its risks as one of the helper columns
must contain the worksheet's row number as a value (and this can get out of
synch easily after inserts/deletions/sorting).

If there are no better solutions to the ITD formula than what I'm currently
doing, any suggestions for a function which will simply return the row number
as a value in a cell?

TIA.