Intermittent Fasting help

I'm a newbie to intermittent fasting and I just have some questions! I've been following mfp for a few months now and I'm just stalled and getting bored of it.

I plan to do 5:2 which is like "fast" for only two days a week (eating 500 calories on these two days) and eat normally on the other 5. and join it with the 8- hour diet so like fast 16 hours a day and eat during the other 8, but how do I calculate how much I should eat on my non fasting days??

Like should I eat at my bmr x activity level, my tdee, or the number of calories mfp tells me to eat? Or something completely different? I'm just a bit confused on the whole process! And if anybody has had any luck with fasting like does it really work? Thanks!