Expressions question

I'm working on doing a little film restoration for one of my film professors. I'm working on a luminance macro that will take the average luma of a portion of a frame (selected using crop and blur for lows, mids and highs), calculate the difference from the target luma and reapply that via probably an expand node and gamma. I've got limited experience with expressions other than direct linking and some basic math... is there a primer somewhere to let me know how to grab the luma value of a specific pixel?

TIA

Cole McDonald
YAFI Underground