You want to "masterbatch" your lye, from what you are describing.
There is an option for this in the software:
You slide the slider to "yes" and then put in your water:lye concentration (by weight, even the water- all ingredients are always by weight, not volume).
The soapmaker decides the best amount to use, as some want to use the full water in all recipes, others may want to add goat's milk, aloe gel, etc.
If you use a 50% lye solution (1:1), just be extra careful because that's very cautic and can burn if it hits bare skin.
Use a sturdy plastic container with a lid- think big washing detergent container or the like- and make sure there is some headspace.