Don't just arbitrarily change needle size AND do the next size larger. First you need to know how many sts per inch you get on which needles so CO about 24 sts and work 2-3" with the 8s, then switch to the 9s and knit another 2-3". Decide which needle makes a fabric you like and how many sts per inch that is. Then divide that into the sts for the next larger size and see if that will give you the right finished measurement.
Since it's top down though, you can also CO the sts for the next size larger and only do the increase rows you need to get to the size you want. Try it on when the back sts measure about half the finished measurement you want and see if you need to do more inc rounds or rip out a couple - you want the raglan inc lines to almost meet at your undearm.