John wrote:
XL says it will not work in merged cells... anyway around this?

I would get rid of merged cells if I were you. If merged cells is
absolutely critical to the formatting/presentation, a work-around might
be to do the calculations on a worksheet with no merged cells, then look
up the values on a formatted worksheet.

BTW "merge and center" can effectively be replaced with no merged cells.
Select the cells in question and apply Format | Cells | Alignment |
Horizontal | Center Across Selection.