Time, Grace and Morris.
There is plenty of debate on tinternet about the amount of fasting calories which provide the optimum health benefits.
Lots of the body building sites seem to go for more like 700/800.
The one thing most of them can agree on is that if your body fat low (Lower than 16% for women and 12% for men), then you shouldn't eat LESS than 500 or 600 cals as you lose muscle not fat.
Exactly as Bigchoc says, you're still creating a significant deficit.
I still think you'll experience good health benefits at that sort of level.
It's far better to come in on 700 or 800 cal regularly then feel you've failed and stuff yourself with biscuits (classic "diet" cycle).
Personally doing 5:2 since August 2013. My blood pressure was sky high. It's now completely normal and actually very good for my age (40). My resting heart rate was about 80, its now 59. 
I have and do plenty of "FD" which are like mini fasts. Ie 700/800 cals.
I have 2 small DC, a DH who is never home. So with everything that goes on, it's often hard to eat well. Tiredness stress, endless child illnesses, up all night etc etc is usually or rather used to be a recipe for diving headlong into the biscuit tin.
It isn't any longer. And I eat far healthier. Simply because the healthy stuff ie lean protein and veg lets me have a maohssive portion on a FD.
You will still see good weightloss and results.
There are lots of very experienced fasters on this thread (Betsy, Bssh, Breadandwine, Bigchoc, Earslaps, Eatriskier
). And they have worked towards liquid only fasts and fasts below 200 cals. I don't think they started this way.
I am a wimp so I did 16:8, then under 1000 cals and worked my way downwards. I've done a few LOFs but I've never ever been under 400 cals on a FD yet. Perhaps later when I'm more used to this woe/wol.

I have a cold just to add to my misery of sick DC. Feel dreadful. Still doing my FD but will be a mini fast of 800 cals.