Ya, I'd think 50% is standard for C&S nurseries. IMO, certainly the max, even in AZ. We did go with 60% on the first shadehouse, while it's ok when fully exposed to the sun all day, however if there is any shade from a tree or whatever, it gets pretty dark. I'm going to change it to 50% next year. 10% may not sound like much, but it is.
At home we use 30% over the GH and 40% for the shadehouse.
The higher the percentage number, the darker the cloth.