Proper Rope Clutch Size

[Tigi Too]

Hello!
I am new to the forum so if I am making any forum faux pauxs please be patient!

OK we have a Newport 41 mk II and we have been using Spinloc XTS triple rope clutches. The halyard (10mm) has always slipped and now our Asym Kite halyard (10mm) is slipping. I suspect the jib halyard is slipping but it is not as noticeable. After numerous emails with Spinloc (they have been as helpful as they can be) I am trying to ascertain if I need to go the next level rope clutch?
It appears one big question is what is the load at the clutch - if I can figure that out I can then spec out the clutch - perhaps going to a Lewmar D2.
So any thoughts-suggestions-help would be greatly appreciated! Thanks.
This has come on the heels on seeing a friend's recently purchased J122 and he seems to have bigger clutches than mine.
Dieter Hugel