We'll look forward to hearing how you feel about it. I really enjoyed the sensations at 5%, and probably would be fine with carefully measured greater small increments, perhaps 6-8%, which I'd like to try sometime soon. Have fun.
Yes, I went from 5% to 7.5% and didn't find a lot of difference, but that may be me. I found that 10% gave about the same response as a fairly soapy enema at 15-20 grams per liter. For a 3 liter soapy enema (the maximum volume I typically use for soapy enemas), that's half to two-thirds of a bar of Ivory soap. The soap only enema "dissolves" more in the colon, but the reaction for glycerin only at 10% is about the same as the plain soapy enema.
I get a stronger reaction at 15%. The contractions are stronger, more frequent, and get progressively stronger making it harder to hold. At 20-25%, I get much stronger contractions and they do start feeling like cramps. I can get incipient nausea for a period of time and it's a good idea to have a trash can nearby. A real possibility of blowing out of both ends.
My "go to" soap glycerin enema is a 3.0 liter. It's a mixture of 300 ml of glycerin dissolved in 2.70 liters of very warm water. Although glycerin is soluble in water, its viscosity requires some small effort to dissolve it evenly. I also add 30 grams of grated Ivory soap to the mixture as I am mixing it. It gives it a nice milky color (which you can see in a clear bag) and a soapy head. I may not take it all, but it is there just in case I want it.
I find I usually have a reaction to the combination in the first few minutes that provide relatively strong contractions in the descending colon as the enema solution flows in. It then tends to settle down before building up again to a series of periodic and rhythmic contraction waves rippling down my colon. The volume is great enough to get the enema all the way into the transverse colon without being so filling (for me) that there isn't space for the enema solution to move with the contraction waves.
I'm usually able to hold this mixture between 15 - 20 minutes before I seek the relief of releasing. On occasion, 30 minutes is possible. And once (and only once) was I able to hold it beyond an hour (actually 84 minutes if I recall correctly) as I was sharing a very erotic fantasy with another Zity member as she coached me through holding it to an hour. Once I got to 50 minutes, I knew I was going to make it to an hour because the contractions were more or less continuous and rippling but not "strong." And then it was a matter of trying to keep going until I had an explosive orgasm. It felt good to release and my muscles were exhausted from the continual contractions. I haven't been able to replicate that experience on my own.