Shoulder strength seems to be the main factor for achieving a human flag. I was wondering if there is some number relative to the bodyweight that indicates when one's press strength is ready to try a human flag. E.g. you should start training the human flag on a bar when you can move 0.7 times your bodyweight in a standing press. Or even: You need at least 0.8 to do a human flag.
Jim Bathurst, known for his web site Beast Skills, says in his human flag tutorial:
Overcoming Gravity, a popular book on gymnastic training for adults, has a chart comparing the difficulty of various skills. In the chart, a full human flag is a level 8 skill, which is "equivalent" to a press of your bodyweight, or weighted dips with 85% bodyweight added. The first progression to a flag, the tuck flag, is level 5 skill, equating a press with 0.65% of bodyweight or dips with ~37% added.
Both of these sources are at best anecdotal, so take them with a grain of salt. My suggestion, if a human flag is one of your goals, is to start training specifically for it as soon as possible. If you can hold the tuck flag for at least 10s, then add it to your routine and practice!