Since most chassis cars have critical geometry as it pertains to ride height and bar angles, we tend to change cross by adding a slight amount of ride height to one diagonal and removing a slight amount of ride height to the other diagonal. That keeps the car at the correct attitude, the bar angles correct, and only changes one variable.
So, on a car which is properly set up in the first place and needs a certain crossweight percentage change, I vote whole car.
If the car isn't correct to start with you may need to favor one diagonal or the other, or perhaps even just one corner. When Tony Stewart rolls into the pits in his Sunday ride, one turn in the RR changes that percentage without considering the whole car but it's only a few thousandths ride height and a minuscule percentage.
ramey36, thank you very much for explaining about bite and where it came from.
I've either been taken to as a kid or gone to dirt races now off and on for around 60 years and been involved trying to learn about how things work for over 20 now. It's the sort of thing where no matter what you learn there's always more you do not know about, which shows itself to you. What you explained about where thoughts on rear bite came from, as far as inches of bite, I'd never heard before. Thank you again it makes perfect sense to me.
I understand your thoughts on, "If I can't get forward traction off the corner with 50 lb bite, I want 75 then test.". I especially like the if 50 ain't enough then put 75 to it and >TEST<. For me it's all about put your best guess on the track and after that it's about solving on track problems.
Keeping with the original question I made then it sounds like there are three general ways most look at changing cross.
The first is to put turns in the back adding or taking out cross. Or go to the front to do it depending on circumstances.
Next some look at dealing with the diagonal as a whole, looking at splitting adjustments made across the whole diagonal
And lastly I see some when looking to make a cross adjustment, immediately considering the whole car and wanting to split any adjustment around all four corners.