I make Laundry Soap all the time. I do the dry/powder recipe rather than making the liquid kind. I think the liquid kind is more work than I'd like to deal with, and I've heard that it can separate over time - that it needs to be occasionally remixed?
Anyway, my thought is that if the recipe is exactly the same minus the water that's fine enough for me, and should result in a more concentrated detergent! The water will always get added in the wash anyway.
I just do 1 bar Fels Naptha, 1 cup washing soda, 1 cup borax, and 1/2 cup oxyclean or other oxidizing agent. I shred the bar of soap in my food processor using the grating blade, then I put it through a second time using the steel chopping blade until it's super fine. Then I add the powdered ingredients and pulse a few times till it's well mixed.
I'm sure I'll eventually burn out the motor on my food processor, but I'll cross that bridge when I come to it!
I only have to use 1 Tablespoon of detergent per load - slightly more if an extra dirty or extra large load. I have an HE washer, use twice as much if you have a standard washer. I actually believe I could use less detergent than that, as much as half of what I use, but I'm so used to using extra detergent due to dirty kids and hubby who works in a dirty job.