Hi Russ - I am far from being a
sailmaker, but I bet your mono experience is a good reference for where to put add'l
reefs. Despite the "proper" locations, the build of your sail will probably dictate the available locations considering batten and luff car (if you have them) locations. Only your
sailmaker knows for sure.
I have three
reefs. I don't know the exact % but the second is a bigger chunk than the first and the third is bigger than the second. By eyeball measurement, by the time the third reef is in, a little more than a third of the sail is left. It's logical to me that the third reef should represent what you want for a storm sail. I've never used the third and don't plan on it.
As for "don't leave home without it" I'd put my sym spi on the list (after main & genny). I'm currently in the process of acquiring a Code 0 to round out my inventory.
Hope this helps,
Dave