Heel of fully extended leg on the pedal is a good starting point, much as it is on an upright. (KOPS is right out
)
From there, it's trial and error: Your pedalling technique is liable to be different in the recumbent position (I find that I ankle more, especially if the bottom bracket is relatively low). Also most riders tend to settle in the seat after they've been riding for a while, so the optimal extension half an hour into a ride might be a little bit longer than when you first get on.
Depending on how the seat is attached, changes in the seat angle may or may not affect the required boom extension. HPVelotechnik style mountings pivot the seat around your hip joint to avoid this (ostensibly to simplify on-the-fly seat adjustment for different styles of riding), but most will pivot around the lower seat bracket.
As on an upright, if your knees hurt, the boom is probably too short. If you feel overextended, get Achilles pain, or find yourself sliding too far down in the seat, it's probably too long.