Maths question

I'm sure someone must have done something like this before (i'm sure i have too at some point but cant remember), anyways. I'm working on a project for home (no i'm not a student) which involves using an analog to digital converter hooked up to my PC. Now I want (in visual basic) an analog gauge and a digital gauge (the analog is done).

OK, my AD input range is 0 - 255, I want to make a value of 0 to return a value of -20 and a value of 255 to return a value of +20, therefore a value of 126.5 to give a value of 0. I can do this if it were ranging from 0 upwards as it's just straightforward division but havnt a clue how to do it where the returned values are +/- figures. I just cant figure what maths are involved to return the correct values.

Any help would be greatly appreciated.

(ValueIn * (Scale_Max - Scale_Min) / (Raw_Max - Raw_Min)) + Scale_Min

Check: (128 * ( 20 - (-20)) / (255 - 0)) + (-20) =
(128 * (40 / 255)) + (-20) =
(128 * 0.15686) + (-20) =
20.078 - 20 = 0.078

[edit] little mistake, this should give -20 till + 20

(analog input/6.4)-20

[/edit]

Just put it in, and may i congratulate you sir, spot on with the maths.

Thankyou very much Rdrast