I had a see-saw with my weight gain all my life
I went up to 128 Kg and came down to 84 Kg
Again went up to 123 Kg and now maintaining 90 Kg for last one year.
From my experience there is only equation for waight gain

Calories taken in > Calories spent = Waight gain
Calories taken in < Calories spent = Waight loss
Calories taken in = Calories spent = Maintain weight