My uncle helped develop the bonzers way back and he gave me one of his old bonzers. He makes his own fins that look like a greenough with a slightly wider base. On the 7’8” bonzer he gave me the fin is 6.75”. I think placement is really critical as well. He put some marks where I should run the fin and it’s a small margin maybe 1” or so where he says it works.
I will also say I have barely used my bonzer so I can’t coment on personal experience just what he told me when I got it.
To get to the point I think 7” greenough is pretty close to what I have.