If you like Purina products than simply half the sack you have now with the Purina goat chow for milkers. When you run out of the medicated, she will then be on straight chow. Adding BOSS is fine, it's high in fat.
A doe who just freshens is milking alot of calories, so feed her well. All the alfalfa she wants, minerals, lots of clean water, and grain on the milkstand when you milk her twice a day.
Be very careful with your horse information, they are single stomached hay burners

goats have 4 chambers of that stomach and too much grain or too much oils, too much molassas and you have acidosis which can kill your doe.
If you are feeding her 1 pound of grain a day, take a week to slowly build it to two, another week to slowly build it to three...a good size doe will eat this much for maintence and even more to stay in milk.
Young milkers are the hardest group to keep any weight on at all, they milk off everything you give them...my 3 are 14 months old and all three are milking over 8 pounds, being 125 and 135 pounds they are thin....really thin for my herd

But lean is not a bad thing, it's soo much better than fat.
Always feel over the ribs of your doe, if you feel just ribs than yes you are right she needs more calories, if you feel her ribs but there is some flesh on them, than pregnant or not she is perfect....now grab a handful of fat over the lower ribs, shes fat. Vicki