Bp is the way to go friend. you can use that in sex no problem, and it will make sure you have an accurate measurement, other wise you run the risk of inflating your progress or being sorely disappointed.
Also you have girth for days :) you can worry about that after, it should be no problem to get to your girth goal after lenght goals are achieved. (You will most likely gain girth as you look for lenght as the two often come hand in hand)
good luck :)