This is what IrishLass posted for master batching and it works great for me.
make my 50% lye solution in a plastic PP #5 pitcher with lid, and when it has cooled down I store it in an airtight HDPE container which happenes to be a cleaned-out, reclaimed laundry detergent bottle with a no-drip spout and screwtop lid. As long as it is stored in a proper container and tightly covered, it lasts for a very long time. I'm talking much more than just months, but years.
The reason I like working from a 50% solution is because it's super easy to figure out the math that it takes to tailor it to use with any recipe no matter how big or small the batch, and no matter what % lye solution you choose to employ for your batch. To tailor it to whatever lye solution I choose to use, I just use this simple 2-part equation, which works across the board with every formula no matter what:
Here's how much 50% lye solution to weigh out for your batch: Multiply the total recipe amount of dry lye your formula calls for by 2. That will give you how much of the 50% lye solution you need to weigh out for your batch.
Here's how much extra water to weigh out for your batch: Subtract the total recipe amount of dry lye for your formula from the total recipe amount of liquid for your formula. That will give you how much more liquid you need to weigh out for your batch to compensate.
That's all it takes to figure it out. Easy-peasy.
Now, I do need to make mention that there is some slight evaporation that takes place when mixing the 50% solution. After my very first master-batch in which I made sure to weigh everything from beginning to end (container/cover and all), I figured out that my water loss from the heat reaction during mixing came to 6 grams worth, i.e., it weighed 6 grams less than it should have, so I just added 6 grams of water back in to compensate and all went well.
The size of my 50/50 masterbatch is a little over 4 lbs. worth, so as a result of my first experience, after having first weighed out my container and cover, I now weigh out 2 lbs 3.3 oz (or 1002 grams) of lye. And then I weigh out 2 lbs 3.6 oz (or 1008 grams) of water- a 6 grams excess. In the end, right when I get done mixing, I weigh my finished solution to see if it all adds up correctly (the weight should be 2004 grams if all went well). If it comes up short (which surprisingly has never happened yet with my subsequent masterbatches), I would just add more water at this time to equal 2004 grams, but so far, all has gone well and it weighs out 2004 grams - an even 1:1 ratio. When I go to weigh the solution later, after it has cooled off, it's still 2004 grams (I cover my container as soon as I'm done mixing).
