Question: With your slab reefing system, do all your clew reefing lines enter the boom at the aft end and, if so, how do you get proper tension along the leech and foot, particularly on the last reef?
Answer: In our case, the clew lines enter the boom through sheave boxes at the correct position in relation to the clew, so no problem. However, there is a simple answer to your problem of maintaining the correct angle when all lines exit at the boom end as long as you don’t mind changing your single line reefing system: The key is where on the boom you dead end the line after it runs through the clew reefing cringle.
The easiest way to get this right is to tie the line around the boom with a timber hitch and then slide it back and forth until you get the correct position. Once you are sure you have it correct you can add a rounded pad eye to the boom at that position so that the timber hitch won’t slide around. It’s important to understand that the pennant still passes around the boom with a timber hitch and the pad eye only acts to stop fore and aft movement.
The only time this won’t work is if your mainsail foot has a bolt rope into a track, but this is a poor way to do things anyway. If your main is set up this way I suggest you get your sailmaker to add slug slides to the foot. You don’t need a lot of slugs since the strain is all taken by the clew; one every 30” is plenty.