The basic formula for decreasing evenly is to take the number of stitches you have now (110) and divide by the number you're taking away (13). Of course, most of the time, this isn't an even number -- in this case it's approximately 8.5 -- so you would probably want to alternate between decreasing every 8 and every 9 stitches.
(...when I say that, I don't mean "do 8 stitches and then do a decrease and then do 9 stitches and then do a decrease"; you'd be decreasing within the 8 or 9 stitches, so either six stitches plus decrease or seven stitches plus decrease. If that makes sense. Which it probably doesn't because HI I'VE HAD COFFEE so I'm a little bouncyhyper. cough)