Shade Cloth percnetages defined and questions

[Uechi]

I live in Clearwater, Florida and we have plenty of light too much in fact. I have my orchids placed in several areas in my screened porch, but it is still way too bright from 11:00 am till 3:00 pm. I have decided that shade cloth will do the trick. The problem is the more I read about it on the Internet,the more confused I get. One source says 70% shade cloth, for example allows in 70% of the light and blocks 30%. I read another source and they say 70% shade cloth blocks 70% and only allows 30%. Which is it? I was planning on buying cloth that blocks about 40% of the light. Is this to much or too little? Several growers use 50, 60 and 70%. I understand that various species require different levels of light ,but I would think there is a compromise percentage. I also would like a recommended source/s. Home Depot and Lowes, at least around here, sell the same brand and it doesn't really tell how much light it blocks or lets in. Thanks for the help.