There's good advice here about tdees. I'm going to suggest a slightly different approach.
While you must count calories on fast days to ensure you stay below 500, I don't think you should calorie count on non fast days. I don't and have never calorie counted. Calorie counting is something that gets really boring and is why many people give up on diets quickly. Michael Mosley didn't want to do it and that's why he thought 5:2 was such a good solution.
However, it is difficult to know what normal is. as let's face it, we all came here because we want to lose weight, hence, we haven't been eating 'normally' before - either dieting or overeating. However, we all know the difference between bingeing and having a day of eating to satisfaction but not going overboard. So be mindful of what you eat on non fast days at first, but eat when you would normally eat and eat the things you like. If someone offers you something and you want it, take it. If you find you're binging you may need to follow the tdee advice for a while though.
If you restrict too much on non fast days you will end up bingeing or giving up because 'starving' or restricting yourself too much is hellish and doomed to failure.
You asked, what's so different about fasting? Well, it's this: fasting 'resets' your hunger after one to two months. You naturally start to eat less and make better choices. This has happened to me even though I didn't expect/ intend/ try for it to happen. I also now eat a much more balanced diet. I'm not interested in biscuits and sugary junk no longer has a hold over me. This probably sounds crazy, but it's true. Your body just sort of adapts and casts off old bad habits. I've lost a stone and a half in 3 months and I've never dieted before - couldn't stick to anything. You obviously have better willpower so you should find this a cinch. The ability to relax slightly on non fast days should make it even easier for you.
You also must realise fasting is something you will do for the rest of your life. It's not like other diets where you do it for 6 months and then spring back up to pre diet weight. But the thing about it is that you'll quickly understand why it will be easy to do for life. I've experienced even more benefits beyond weight loss and resetting hunger so I no longer feel deprived. A new sense of wellbeing, positivity and enjoyment of life, clearer skin, improved memory - I'm still me, but I'm a different, better person.
So I say don't calorie count or aim to stop calorie counting very soon. Otherwise your turning a very special and powerful life journey into a bog standard, doomed to fail diet.
Best wishes. redhead x