Intermediates on a single spreader mast?

[Newsong]

I recently bought "The Complete Rigger's Apprentice" (second edition) in preparation for replacing the rigging on my Cape Dory 330 cutter.

My mast has single spreaders and aft-led intermediates. I like the idea of moving the intermediates out onto the spreaders and adding running backs.

In the book figure 5-25 gives the percentage of maximum shroud load allotted to each shroud. I did not see what percentage to use for intermediates on a single spreader mast.

What is a safe percentage to use to size these shrouds?

I'm sure the intermediates take some load off of the uppers, is there a good way to calculate how much? (My calculations have my uppers right at the top end of 1/4 wire. I am trying to decide if I should move up to 9/32.)