#TL;DR
- given that the "ideal" numbers are +255, +255, +255
- adjust screw A til #1 is close to +255
- adjust screw B til #2 & #3 are close to equal
- this may require adjusting screw C if #2 or #3 start +ERR-ing
- adjust screw C til #2 & #3 are close to #1