10 to 15% is not an absolute. I can only go to 500# so I aimed for @ 400#, as long as I don't get any trailer sway I should be OK. I can always get a little more if necessary, but it doesn't take much to increase or decrease the tongue weight.. Below is a link to a page from Etrailer.com that I used to weigh the tongue weight, drop down 1 page. I did put a grove in both ends of the 2x4 to make it easier to set the pipe in the right place. I used 4' between the pipes to make the math easier.
I imagine the scale could be off a little, so I'm not going to try to get the weight to the exact #.