Super easy keeping broodmares are a challenge, and I have one who is a super-fluff, so I hear you on the difficulties of how easy it is to over-feed. She needs enough volume to satisfy the chew urge all horses have and settle her appetite, but not so much that she explodes. It’s a very tight line to walk.
The best thing you can do is read the labels of your balancer. If it says that it is nutritionally formulated for pegnant and lactating broodmares, then you are good to go. Just start increasing the protein in her diet - and in your case it would be better to do this via a protein-sourced hay. It may take some tweaking to keep her from either losing weight or blossoming into a potato.
I feed timothy to my easy-keeper, which is 10-12% protein, plus timothy is fairly balanced calcium : phosphorous. As she gets to day 270, then I might add 3-5 pounds of alfalfa and this provides a bump in protein and adds some extra lysine. If she starts turning into a potato, then I may reduce the timothy, but maintain the alfalfa, or sometimes what I do is start taking the mare for walks in hand to increase her exercise so I can continue with the same volume of hay. This way we both get our exercise. 
As long as she continues with a nice layer of fat over her ribs, she’s doing well. You don’t want her losing weight at all, ever, this stage in the game. Even if she is overweight, just work to maintain. The lactation demands after baby is born will automatically cause weight loss. In the meantime, the pregnant/lactating mare balancer will continue to serve as her mineral/vitamin source.
