Complex day of the month calandar
 

I am creating a tool for my work. It would essentially function as a sales
tool for forcasting to the end of the month. My plan was to map out the days
of the month to 31, and have a forcasted number pre-fill for the user. The
number would be based on a beginning number (say 30 on the 1st of the month).
Then, the following days of the month would update with the needed additive
increase to reach the desired ending number (say 60). I have the math
working, but need to find a way to pre-fill the day of the month with the
current number entered into an unlocked cell. (user cell for input) I am
thinking an "if/then statement" is needed, but can't help but think it may be
the long way around.