Shade cloth or shade screen? What %?

[mimigirl]

I am making a covered shelf to summer outside the few orchids I have. Some are high light, dend etc, some low light. I can move the shelf about to change their placement. The spot i have receives morning and early afternoon sun. Again I can move the shelf about. What per centage of blockage am i supposed to use? I live in new orleans in zone 9. Also which do you suggest shade adjusting screen or cloth? I hope i have explained myself well enough.