How to level uneven ceiling joists for sheetrock placement
Sister with steel stud using a lazer. Lazer a line across at your lowest point and sister on the line. Steel stud will stay come straight, stay straight throughout the installation and after the installation. It's easy to notch around existing obstacles and easy to work around with anything new going in.

This is actually a perfect example
 
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If you don't have a lazer a string works well too.