I'd go with what you feel works for you while at the same time being safe about it. You don't want to lower it too much because that can put you at serious risk. Do what you know will work for you. If consuming the amount of calories that you are right now is working just fine then stick to it until you start to have problems. I see no reason to change if what you're doing is working.
Lyle McDonald published an article in which he reviewed some research to the effect that fat can be mobilised from our body at the rate of 31 calories per pound of fat per day. The implication is that lean people should only have small deficits because there is only so much fat that they can mobilise per day, but people with a lot to lose can be OK with much larger deficits.
I hope that you are being very careful with exactly how you use your 600-900 calories. Focus #1 would be some very lean protein (I am tempted to say 150g but that pretty much uses up all your calories) and then vegetables. You will want to take fish oil capsules (essential fatty acids) and possibly a multi-vitamin too.
You will want to keep your plan under review - for example if you stop losing weight, or if you become overly-lethargic or if your hair falls out or whatever then you will want to re-evaluate your plan but if none of that happens you just keep going. Also if you have made significant progress then eventually (when you get closer to a normal BMI) want to start to very slowly raise calorie intake.