Anti-freeze is funny stuff. 100% transfers heat very poorly. Pure water is MUCH better at cooling. There is nothing wrong with diluting antifreeze from traditional 50/50 a little bit...and 30 to 35% is OK but flirting with the limits. If you go less than 25% and the stuff can really degrade and basically turn to jelly, so be cautious you don't accidentally dilute it too far or you'll have a real mess on your hands. Check out page 25 of the link below and see what an 11% mixture can look like...of course that pic is ethylene glycol...not propylene like our normal green stuff is...so not apples to apples...but they can react similarly.
With respect to the green stuff, 50/50 is basically the safest and best overall mix. But anywwhere between 30% and 70% works fine without risk of gunking up the works. 30% will cool much better than 70%. But if you want absolute MAX cooling performance, pure water will always beat any mixture. As long as you have no objection to the corrosion.
