QUOTE(Liridona @ Aug 25 2008, 12:18 PM)

To tune to a standard A I would need to play the F above it. To tune to a C I would play a G.
To tune to a C play an F, to tune to an A you need to play a D - you were right in that it is four degrees removed, but i'm not sure how you got A-F and C-G.
If you play a 'C major' scale on a G instrument, the pitch will be of a G major scale (hence 'in G'). So it can be imagined to be adding a sharp to the scale (or taking away a flat) - if you play D major it will come out as A major, Eb major as Bb and so on.
If you're playing from a part written for a C instrument, take a sharp from the key signature (or add a flat) and there is your new key - it will be raised by a fourth (or lowered by a fifth if you prefer)
so the passage C-B-A (the last three degrees of C major) you would play F-E-D (F major), and F#-C#-B (D major) would be B-F#-E (G major)
basically the note you finger sounds down a perfect fourth, so transpose up the same amount.
hope that made sense!