I can't give you numerical answer, but I will say.
It might carry a lot more stress than you think.
If it fails the whole deal is over.
Don't underbuild it.
I've built several biplanes, pretty much the same deal, except you have to wings to brace against.
I recommend starting low with a lot of line rake. After a test flight you can bring it into trim. That beats starting high and forward and not having anything to trim.