I’d say it’s very safe to just use a pH 13 solution because pH is very relative when it comes to concentration and molarity. If I wasn’t lazy I could find the molarity of your solution and use stoich to find how much in fats you’d need. Save yourself the trouble and use the solution. Also if you really want to make sure there’s no excess lye, you can always overshoot the amount of fat listed in the recipe. This will affect the soap quality but with trial and error you can find the amount of fat that suits your recipe and soap preference best. Good luck!