Increase or decrease calories

I wished it would have been a easy answer for me. I went up and down in cals for 2 months before I finally went up and started eating my TDEE minus 20% and now I am losing!

When you plateau, how do you know when you need to decrease or increase your calorie deficit? There has to be a better way than trial and error.

Depends on your mindset. If you're an evidence based person you might look at the results of controlled experiments and see what happens there. For an obese person this would lead you to go on a low calorie diet for 4-6 weeks as the evidence shows this to be most effective.

Or you can listen to a bunch of randoms on web forums with arbitrary minimum calories and other quirks and try to figure that out, by trial and error.

Good luck ;-)