What is the formula you use for the "singleton" cells? If it's something on the order of:
(where adjust is the cell with the adjustment value), you should be able to apply it to the min-max ranges with something like:
Code:
=TEXT(150 + adjust, "£0") & "-" & TEXT(175 + adjust, "£0")
Granted, this does make the cell contents text, but I don't know if that matters.