I find that the unscented Dr Bronner's is almost irritation free (you can feel that it's not just water, but it's more tingling than irritation), so that might be an alternative. I use 1 teaspoon per quart, which produces very little cramping, but a "stronger" and more complete expulsion for me. I always take a rinse enema afterwards.
Lately I've taken up Dr. Bronners unscented. I generally don't bother with precise measurements and just give the bottle a good single squirt into the bag, swish it a bit and bob's your uncle. If I had to hazard a guess I'd say the solution ends up being on average 1-1.5 tablespoons per 2 quarts of tap water. everybody's preferences are different though so play around a bit until you find something that works for you.
I have used 1 ounce of Dr. Bronners Peppermint soap to about 2 1/2 quarts of water with good results, a little cramping and a good clean out. One ounce is about 2 tablespoons which is not a lot of soap. I am going to try 2 ounces and see if that is too strong. I do like the cool feeling of the peppermint soap coming out.
I actually like Dr Bs unscented or Dove for an enema. The irritation causes cramps which I like and facilitates expulsion. I like the tension which they produce.