The 5:2 fasting in the Michael Mosley sense is not intended for weight loss, but for a more healthy living.
But of course, you can easily use it to gain some weight loss advantages too. I do this on a regular basis, and it works great.
I would suggest that you go with two days of 600 calories (4-500 if you're female), and simply go with your regular RDI on the other days. You will feel AWESOME - the "fasting days" will do that for you - and it will help speed up weight loss.
Initially, doing days of 600 (or less) calories can be quite a challenge. However, it gets a LOT easier as you get your grip on this.
Today, I have scheduled less than 200 calories for the entire day. No biggie.
"Losing weight is never about eating as little as possible"
"Eating enough, moving more, not eating stupid s**t = losing weight."
"Do. Or do not. There is no trying."
- Master Yoda.
I started my weight loss journey on October 4th, 2010 - and this is what I have accomplished so far: