For 230 ml of Cetaphil lotion, I add 0.15% citric acid and 1 gram of Salicylic Acid (to compound a 0.5% SA lotion). One gram of Salicylic acid dissolves in 1ml of USP Glycerin and 2ml of 100% Isopropanol, heated in water bath to 130 degrees, and then the SA will not precipitate out of 200ml of lotion, which otherwise does, without the Glycerin. This body lotion has a measured pH of 4.5, using a pH test strip.

Citric acid is water soluble, and has a very low pH, and costs $8 at Ace Hardware for 7.5 ounces.

Here's a discussion of the difference between SA and AHA: http://dermatology.about.com/cs/skincareproducts/a/aha_2.htm

I like SA because it's effective for exfoliation even at low percentages, and it costs only $15 for half pound if you mail order it, and I can't mail order pure AHA --I think because pure AHA is explosive at high concentrations.

I'm not saying SA is better than AHA for you, but I thought my example might be helpful.

Oh, I was overthinking it. Just take a 1.5 ounce shot glass, and mix a small test batch at whatever ratio you are planning to use. It the Cetaphil doesn't turn into water instead of a cream, or turn into clumps, or turn slightly brown-red, probably nothing reacted and the mix is ok.