Dr.Mosley found that the research showed that the longer the period you leave between your last meal & breaking your fast,on fast days, worked very well i.e. 16:8 (optimum window between 16:8 and 20:4, anything longer might not be so effective in the long term). However, he couldn't do without breakfast! Hence he leaves 12 hour gaps. I have found leaving a 16 hour gap more effective for me.
As others have said you need to check out your calorie intake on non-fast days, your TEE. I think somewhere between 1400 and 1600 might be right, but go to the Progress Tracker and check out your own details. And on fast days you might need to eat only 400 calories. I am post menopausal and my body needed this kick!
There is no magic bullet here, just a hard slog to be slim again. Trying the above isn't easy and it isn't for everyone, but try it for 2 weeks and see if it works.
Good Luck....