I researched the values for stiffness on fibers once. can't find the data now, but I do remember there was a lot of variation in reported values. being that plants are harvested at different stages of their growth cycle, that would seem to make sense. Harvesting for max MOE may not be a high yeild, but its what one wants for the best bowstring. I honestly don't think that I ever found data the isolated best moe possible, and some data was from fiber used in matrixes and some from fiber spun into various products which introduce more variables.
you should just scout around for nettles, there are lots of patches out there already. If I remember correctly nettle is more work to process than others, or maybe lower yields compared to others. ramie is a domesticated fiber plant related to nettle, if you want to plant seed.