User entry in a cell unless conditions are met?
 

I am creating a spreadsheet for electrical work. I need certain cells to be
data entered by a user UNLESS they meet a condition, in which case that cell
needs to have a formula calculate the number in the cell. (If the cell to
the right of the data entry cell is a 6, then a formula I have written should
take over and autofill the cell with information from another sheet. If it
is anything other than a 6, the user should input the number.) is there a
way to do this without drop-down boxes or other interruptions to the work
flow? I have used drop down boxes and realized that they are a bad solution
because the cell no longer auto-updates on data entry, it must be reselected
via the drop-down box to refresh it. This is very counter-productive when
there are many cells on a sheet, so I would like to avoid it if at all
possible.
TIA,
-Kai