Interesting question in that I think it might be a first for me. And I wonder why.
I guess if it was important to me on my boat I would get winched up there and measure it. You could also measure it from the deck. You can get two sides of the triangle if you have a stiff enough tape measure. And another option would be to check the Rig-Rite site to see if you can find your Kenyon mast with spreaders. All of this in the event none of your H33 friends know the answer.