Disclosure: I can not take credit for this as I read it on this site, but it is the best way I have heard yet.
The person recommended measuring the hitch height unladen. They step on the hitch and remeasure the hitch height. If the hitch dropped 1/2" and you weigh 200 lbs, then the spring rate is 400 lb/in. That ratio will hold for the normal range of motion of the springs (not exactly true for leaf springs, but close enough for this purpose). So, if your trailer weight is 10,000 lbs and you are looking for 1000 to 1500 lbs tongue weight, you want to see the hitch drop 2-1/2 to 3-3/4 inches.
Formula:
Desired hitch drop = (measured hitch drop x trailer weight x 0.10)/(your weight)
Desired hitch drop = (0.5" x 10,000 lbs x 0.10) / (200 lbs)
Desired hitch drop = 2.5"