Thanks guys n gals, I've now cracked it using the method:
captainspeaking2U
In cell C1 add =sum(a1*2)-b1 when you have a value in C1 put your cursor in that cell and press copy, then hi-light the cells you need ie C2 onward, then press paste. This will insert the correct formulea into all the cells hi-lighted.
I need to check out
ExGrunt's method, because that might aviod extra columns containing a common constant, which I'm creating by calculating the constant in the first cell of the column, then using captainspeaking2U's method to duplicate the calculation (albeit giving a constant result).
Main thing for now is to crunch the numbers (now grown to 2100 rows and ever-increasing) to meet a publishing deadline, so many thanks for all your help.
This feedback should help others who need this technique: the above quoted method works.
