Item #9 on page 24/25 is the BLFC... http://www.pentairwatertreatment.com...al%2042775.pdf
I'd say to know for sure, you'd have to capture all the water sent to the brine tank during a regen (force the 7000 thru all cycles except the brine fill, let that one run while you capture all the water). But I suspect the amount of water is pretty darn close to what the engineers design it for. And if you have the lid on the tank, unless it is very warm in the brine room, probably not much evaporates out. If you wanted to be sure, test your water just prior to a regen, and see if there is any hardness "leakage" because you are running out of brine in the resin the last day before a regen.