Maybe this site will help you know where to measure at the withers
http://www.lgd.org/library/measure.htm
We have been weighing and measuring our puppy since we got her week by week also taking pictures of her weekly. My son is nine and this is his 4-H project on how she is growing. I also found another chart stating at 3 mos. "add about 4" to 4 1/2" to get estimated full Growth Size" I'll have to let you know if this is true or not. Shelly our pup is now 12 weeks (3 mos.)
She is 10 1/2" and weights 7lb 2.8 oz So if that chart is true our pup will be around 14 1/2" to 15" full grown.
We use a postal scale (that goes up to 50lbs

lets hope she don't tip the scale at that weight LOL

)to weigh her and made a wicket (not a real one like in the shows but it works for us) here's the web site we used
http://ezwicket.homestead.com/index~ns4.html
At first we tape a yard stick to a wall and then put a piece of cardboard across her back at the withers but she moved around to much this wicket we made really helped.
It really won't matter either way what size she turns out to be as long as she is healthy. But it's kind of fun tracking her on the charts.
I'm sure someone can interpret the charts better than what I can. But it seems to me being 13" at 5 months (20 weeks) is in the safe zone. Again I'm not sure if being a male or female has anything to do with it or not. Here are some pic's of weighing, measuring height and a pic of the wicket we made.