One theory that works for adults is consider the standard 13.5" LOP is for a 5'10" adult. Then for every inch more or less in height add or subtract 1/8" LOP. Not sure if it works for very short individuals. I don't like to go much less than 12-12.5" and keep in mind the overall legal length. Compare this to the crook of the elbow measurement and go with whichever is longer.

"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli