Here's how that works: Mix your 50/50 solution of lye and distilled water by WEIGHT. Always add the lye to the water. Make sure it is very well mixed. Store it in a HEAVY DUTY, PLASTIC container that is WELL MARKED. (I am not yelling, I am emphasizing

). When you are making soap, lets say your recipe calls for 10 oz. of lye - grab your container, again make sure it is well mixed, and measure out 20 oz.(50/50 mix = 10 oz. lye and 10 oz. water.) Now lets say that your recipe is calling for 20 oz. of water, add 10 more ounces of distilled water to your mix. (If you were going to use 5 oz of another liquid, use that as part of your 10 oz.) Now you can add this mixed liquid to your oils and bring to trace.
Keep this in mind - Lye water is more dangerous to store than dry lye. A spill will cause more damage and be harder to clean up. It is easier for an animal or a child to ingest fatal doses of liquid form lye than dry lye.
Hope that helps - I can't help you with the specifics of using yogurt, milk, or other special liquids because I have never used anything other than water in my soaps.
Check back here for more responses, because I am sure the are more experienced soapers that will chime in on this thread.