Sorry fella had a couple of ciders this afternoon coupled with the fact I don't have a spreadsheet program installed yet, kinda means I can't be much immediate help.
What I would say though is you most likely have the formula already it's just in the wrong order, google 'algerbric transformations' should show you how to isolate any element of the equation and work it.
What I would say though is you most likely have the formula already it's just in the wrong order, google 'algerbric transformations' should show you how to isolate any element of the equation and work it.