Excel 2007 : Use conditional format to change any value entered in cell to a neg num

I'm using Excel 2007 and have a table in a spreadsheet that is used by multiple people and submitted monthly for it to be rolled up to a regional report. In this table, I track opened stores of various types in rows 6-11. In row 12 I track closed stores without any delineation of the type that closes. Row 13 sums up the entries as you would expect ((SUM B6:B11)-B12). The problem is that the number in row 12 has to be entered as a positive and when you glance at the table, it doesn't seem to make sense. I need to be able to change the report to where a negative number is entered and change the sum formula. The problem is that I can't get everyone to enter the number in row 12 as a negative and it messes up the data.

My question is: can I use conditional formatting to convert the number entered in a cell in row 12 (e.g. B12) to a negative number so I don't have to depend on the multiple users remembering to enter a negative number?

I've tried using =-ABS(B12) but can't get it to work so I must not understand how to use this in the conditional format set up box.

Any help would be appreciated.

Good day,

You can restrict values that can be entered in cells in Row12 using 'Data validation' and allowing only 'whole numbers less than or equal to 0'