This isn't the most scientific way to do it.. I never mix a working solution as it does not keep (it will grow mold and get scummy if you keep it).
If you're only doing two rolls at a time, I fill my developing tank with water and stick my finger into the photoflo bottle. I remove my finger and let 1-3 drops of photoflo drop into the developing tank with water. I then stick my reels with film into the tank and gently spin them or agitate lightly to mix the photoflo. Works for me. The residue could be deposits from your water. Where I live the water quality is all over the place. I've also gotten streaks with photoflo that has been mixed too strong.
When you add the drops, notice the surface of the water and how the photo flow spreads. Once it starts looking like soap water, if it starts forming bubbles or looking like you could wash dishes with it, you've mixed it too strong. You want to just break the surface tension of the water.
Hope this helps. If anyone does it a better way, or if i've been doing it wrong for the past few years, please let me know.