Hi Josephine, if they are very different, think about which will make your day easy and sustainable.
If you can avoid counting calories on your non fast day, do that. Concentrate on eating when you are hungry and making good nutritious meals. As little junk food as you can manage!
The main thing is to get your fast days working well. All power to you.

Josephine, your BMR is the amount of calories your body uses if you do nothing but lay in bed for 24 hours – this is less than the “sedentary” category for activity levels. The TDEE is the calories used during a normal day and will vary along with your activity level, height, age and gender. When Michael Mosley wrote this diet he used TDEE as his guide for eating on the NFDs (non fast days) and suggested FDs should be 25% of that. He then went on to suggest calorie levels for FDs based on the average TDEE for the population. This was 500 calories for women and 600 for men.

However Michael Mosley has since updated the book based on more recently published research and suggested that 800 calories is a more appropriate level for FDs and that for quicker weight loss you eat 800 calories every single day and stick to a Mediterranean diet for 2-3 months, then move to 5:2. For the slow road you do 2 FDs of up to 800 calories each week. The recommendation that your food choices follow a Mediterranean diet is now Dr Mosley’s recommendation for everyone on FDs and NFDs.

There are people on this forum who have lost weight using each of these variations, so it’s a matter of finding the one that’s right for you.
I have also tried all of these variations and found that to lose weight I need more than two 800 calorie FDs per week if I’m eating up to my TDEE on NFDs. So if I’m doing only two FDs I stay under 500 calories.
The 800 calories every day is a big commitment and not for everyone, but very helpful for those with a significant obesity issue, or who have weight related health issues that need to be addressed quickly – eg diabetes.

Like Cinque, I do not count calories on NFDs or FDs, although I did in the beginning to make sure I knew my portion sizes. Most people here find that they have a range of “go to” meals and snacks for FDs that they use repeatedly.