Hoist the sail and measure up to each reef point from the block or whatever turns it on the boom and double that. Measure to the winch
or cleat where it will be fastened from the boom turning block and add those two together plus 5' or so for knot
on the boom and a little bit to tail at the winch
. If you reef at the boom, line should only be the length of the boom and double the length up to the sail reef point. With a 5' reef that would be 10' plus length of the boom, say 10' for a total length or around 20' or double the length of the boom. A deeper reef would take more line. My reef lines lead back to the cockpit
so can get a bit long though the tail isn't all that long except the for the third reef line. IIRC, the third reef point clew line is 65' or so with a reefed tail for the 17' high reef of about 34'. 1st and 2nd reefs
are only 5' and 10' deep.
The outhaul moves over a short distance even if the main is loose footed. If there is a tackle for additional mechanical advantage, that needs to be added in. Basically it will be the length from the end of the boom to the sail at a loose setting plus any line in the tackle and then back to where ever it is cleated.