Labatt's suggestion of tying a bowline around the boom only will work if you have a loose footed main or one with slugs along the foot. If you have a main with a boltrope, you'll have to tie the rope to the padeye instead.
Any main should have slugs/eyes in the foot below the position of the reef cringles, if there is not other arrangement for terminating the reefing line, so that the reefing line can circle the boom and tie to itself. My main has a boltrope with such provision.
If you secure the reefing line to a padeye on the bottom of the boom, then the reefing line will not pull the reef clew to the boom, but to one SIDE of the boom. At the very least the foot of the sail wont set right, given all the force on the line, it could have other undesired results.
The way I lead the reefing line from the end of the boom is:
- through the reef clew, starboard to port
- down the port side of the main
- through the eye in the foot, above the boom, port to starboard
- under the boom and up the port side , where I secure its end in a bowline around the reefing line where it enters the eye.
When winched hard, this arrangement should permit the reef clew to be pulled all the way to the boom, touching the center of the boom.
If you look carefully at the diagram and text in CAPTTB's picture, this is how the reefing line is terminated.