Belele, the ‘magic number’ is the 600 calories on a Fast Day. How many calories you consume on a Slow Day will either maintain your weight loss or wipe it out. The number of calories for a Fast Day is not based on your TDEE. Rather, the calories you ingest on a Slow Day should be based on what it does to the numbers shown on the bathroom scale.

Bottom line: if you faithfully eat no more than 600 quality calories on a Fast Day and you lose weight and keep it off during the week, then you don’t need to think about your TDEE on Slow Days. But if you are faithful on Fast Days but continue to gain the other days, then you need to look at what you eat on Slow Days and investigate your calorie intake.

I too started this with not much weight to lose, but lost it and then lost some more as I discovered my optimum TDEE. Am now at a weight I never thought possible and maintaining it.
Good luck.

@fasting_me I love how you refer to non fast days as Slow Days! May well adopt this term x