I know exactly what you mean about 600 being do-able! I’ve always done 600 instead of 500. I’m really active anyway and just found that the extra 100 calories meant that I was more likely to do the fast days. I normally do 5:2 but sometimes 4:3 if things are getting a bit sluggish.

In the past I’ve dropped to 500 calories but haven’t found the weight has come off so easily.