If it is dry and was not served moist or wet then measure and use that as the base for your evening feed. Say you feed 2 pounds and what is left is 1/2 pound - then add the supplements and 1 1/2 pounds - if she is eating it all at night. If this is every day then figure out how much she is leaving and subtract that from that feed. So if she normally gets 2 pounds only give her 1 1/2. As long as she is not losing weight then not a problem - if she is then you may need to look at different feeds for palatability or moistening feed if she prefers that.
Some horse people change their horse, they change their tack and discipline, they change their instructor; they never change themselves.