Thats great that you know how much she weighs right now and you're right she is under. If she has been nursing that filly this whole time on just hay I'm amazed she has as much weight on her as she does! Nursing is the most metabolically demanding process there is. Did you know that the amount of energy needed to nurse a foal is more than that needed by a thoroughbred in full race training!!
Anyway, what you need to do is body condition score her so you can assess how much under weight she is. Check out this link and follow the instructions http://www.umext.maine.edu/onlinepubs/htmpubs/1010.htm
(this is just one of many such sites)
Once you know her body condition score (she should be less than 5) let me know and we can figure out what her requirement is base don her being 900lbs and then we will increase it by a factor based on how much under weight she is.
As a basic rule feed her as much grass hay as she will eat (filly too) and then feed her a commercial feed that is formulated for lactating mares and youngstock. This will be balanced correctly for her specific needs. It will say on the bag how many pounds to feed each day based on her weight. Introduce it slowly increasing by about 1/4-1/2 of a pound per day until you get up to the correct amount. If she condition scores at about a 4 then you can increase the amount suggested for a 900lb horse by about 25%. This way you will be feeding her in excess of her current weight requirement and she will put on weight. (Basically you will be feeding her as oer her ideal size rather than her actual size).
If you feed this type of commercial feed you can also feed a little to the filly. There should be info about this on the bag. She will likely need some additional calories etc to meet the needs of her growth however be very careful not to overfeed her grain as in large boned breeds degenrerative joint disease can be an issue. So feed at the conservative end of their suggestions for youngstock. Feeding the filly some grain now will mean that she looses less weight when you wean her. Its not common for babies to loose weight at weaning due to the stress. However at her age there will be less stress than if she were being weaned at a younger age.
Hope this helps and let me know what her body condition score is.