I usually use 91 percent.. today I tried 99 percent and I will tell you it doesn't always pay to use the 99 percent. It might work better for some types of glues/tapes, but for others it doesn't work as good. It kind of melts the tapes too much and turned the blue liner tape that i use into a gooey type mess. I don't get this problem when I use 91 percent. 91 is strong enough to break the bond for me without totally turning the tapes into a gummy mess. So for certain tapes, I recommend sticking to lower than 99 percent.