Put your numbers into the TDEE calculator and see what it comes out at. I would say that you need to make this a manageable diet for yourself long-term, so if it comes out saying that 25% of your TDEE is going to be 400 calories and that just isn’t going to keep you satified on a fast day then don’t worry about it, go for 500 and make sure you stick to your actual TDEE for the other days and you should still see a benefit, it’ll just take that little bit longer to lose whatever you’re aiming to lose probably. Small price to pay for a diet that you’ll actually stick to forever.