The MSDS sheet for Powermax lists it as 65% butane and 35% propane. That's pretty close to the mixture of the stuff you found. It should work OK, I would think.

I went by the local Dick's Sporting Goods and they didn't have any Powermax, but they did have this stuff:

http://www.msrcorp.com/stoves/fuel_blend.asp

Its an 80-20 blend. It should be OK for warmer temperatures. I'm going to try it this weekend. I may have to adjust the gas flow a bit, but we'll see...

Bob