Certain brands run differently then others. It's dependent and your thigh length and size of your rear. I've heard you add 2 to your western saddle size and that's your western measurement. I'm a petite 5"2 and use a 16.5 or 17 depending on the brand.
You need half sizes as well. I can bet you the most popular size of tree you will find is a 17.5 which isn't an option. If you are getting a saddle(depending on your build of course) getting a 17.5 will be your best bet to keep at a good resale value.