There are lots of ideas and options for reefing a Mirror mainsail, this is the one I use on my gaff rigged boat 15563 - Blyskawica.
When I reef, I don't pull the mainsail right to the top of the gaff, instead I have a deck clip/lacing eye on the front edge of the gaff just above where the head of the mainsail comes with the reef in (I've also marked this with a coloured band). I just use the normal length of line attached to the head of the sail to attach the head to this point. This means the gaff can be fully hoisted against the back of the mast, as normal, so it does not flop around. I've seen (in the past) some cruising sailors fit a 'halyard' to the gaff to allow them to lower the sail in the gaff to achieve a similar thing.
My mainsail with reefing point being tried on a Bermuda rigged boat. Reefing reinforcement patches and cringles all my own work (as you can probably tell).
I have a reefing cringle on the luff (originally this was a lacing eye hole), with bearing surface 3120mm from the head point, and one on the leech with bearing surface 3600mm from the head point. If you have a Bermuda rig, make sure the luff cringle is not too close to the bolt rope, so it does not foul the track in the mast when the mainsail is hoisted un-reefed.
I have two, small, cringles roughly equally spaced between these two points and use a length of elastic with a plastic ball at one end to keep the bottom of the sail tidy. Start the elastic in the leech reefing cringle, go down around the sail and back through the cringle, then along to the next one, down around the sail and back through the cringle and so on to the front cringle (see photos above)
There is quite a lot of information and suggestions for options for mainsail reefing on the Mirror discussion forum here https://mirrordiscussforum.org/Drupal_02/node/475 .
