I don't really know if you can get it consistent to a pharmacological level, or that you need to, with Ivory Soap. Maybe you would be a good candidate for Castille Soap?
I get it pretty consistent.
I keep a bar of Ivory Soap floating in 2qts of water under the cabinet at all times. When I'm planning an enema, I shake that mixture up well the night before, when I start my preparations,and again the day of the enema, an hour or so before I start. When I'm ready to start, I take an old wooden spoon( You can still get them at Wal-Mart, and similar places) that I use for no other purpose, I have a different one for cooking, and I stir the mixture, until it looks and feels even.
Then, since I usually use two bags, one a two-quart, and the other a three-quart, until both bags are halfway full. By that time, I've already put in the Epsom Salt, and if I'm using it, table salt and glycerin.
I get each bag half full, then set one aside, and fill one with hot water, attach the hose, and carry it in the bedroom. I set it on a flat surface, then fill the other bag with hot water, and carry it in the bedroom. I then hang the first bag, set the second aside, and start the pre-enema. That way, the soapsuds, Epsom Salt, hot water, and cold water will mix pretty well, and things will be just right!