So the guitar is a Schecter CSH 12, it is a beauty and I love it, but noticed specifically the G strings on the 12th fret sounded quite out of tune with each other, so I went about adjusting the intonation, only to find myself an hour later baffled. I started with the E string to do the whole guitar, but found that as I got one string perfect, adjusting the octave string would bring the other string out of line again...am I doing it wrong, or is there a trick to it?

The main strings are easy as they are regular screws almost like a tun-a-matic, but the octave strings are just small screws holding a plate down, so I have to detune and adjust by hand (in my case pushing them a little bit) and retuning and so on.

So, ideas?