Here's a schematic of the Cal21 mainsheet. The block and cam cleat you have would be where the becket block on the left of the schematic are, or the starboard quarter of your boat atop the transom.
Thanks Guy for the photo that should be a big help for the OP.
Isn't that pretty much what I said? LOL!
I could not remember the exact placement on the transom not the gunnel.
Courtney is My Hero
If a man is to be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most - E.B. White