Conditional Error Message Once a Range is Activated
 

I have a spreadsheet that has a row that needs to equal 100. For example:

Cell C1 equals the sum of cells C2-C25
Cell C1 must equal "100" in order for the user to continue.

Is there a VBA code that will only display a "Total Must Equal 100" message
once row 1 has been activated? Or would it be better to create the code to
be a 'hard stop' when the use tries to save? Any help would be appreciated.

Conditional Error Message Once a Range is Activated
 

Please disregard ~ not as tough as I thought! :)