Ah, right a problem.

The problem is that I want the adjustment to be applied to last months value. So if last month was 100, adjustment 20 would be 120 for this month. Then if next month the adjustment is 50 it would return a value of 170.

However, for the ranges the cells read 'xxx-yyy' so i would therefore need a way for that =TEXT formula to take the first value only (xxx) and add the adjustment, then the second value only (yyy) and add the adjustment.