I don't MasterBatch for specific amounts of soap...I just make up 40lbs of oils because that is what I can comfortably move around. From there, I have a spreadsheet that contains a list of every mold I own and columns that tell me how much Oils, Lye Solution (ready-to-use) and FO. I then just pick whatever mold I want to use, weigh out my oils, lye and scent and make soap.
When I get down to about 4" of Oils in bucket, I transfer it to a smaller bucket and then make up another 40lb batch. If I run short with what's left in the smaller bucket, I just grab the balance from the larger bucket...rinse and repeat. Never had a problem.
And it's the much the same with my Lye Solution. I simply make enough to fill a 1 gallon jug about three-quarters full and then it get low, I have a second 1 gallon jug and will make up another batch of ready-to-use. And like with my oils...if I run short, I just grab the next jug...never had a problem.