50%, 60% or 70% is relative. Important is the light intensity reaching the plants!
50% in 2 meters above plants gives more light and burns plants more easy than 50% in 4 meters hight. That's why some orchids are placed higher in the shade house and others are placed lower.
Two options to get it right:
1) Experience is one option. It's based on observing your plants. If the plants have dark green leafs they don't get enough light. Medium green leafs indicate good light intensity. Yellowish green leafs are the result of too much light.
2)Buy a light meter and measure the light intensity on the leafs and adjust your shade cloth according.
Ups...., and read this: Cattleya Orchid
The easiest way to compensate winter and summer intensity is to add a second layer shade cloth late spring and remove it early autumn.
Most commercialized Dendrobiums can take more light than most Cattleyas.
Hope this helps to get it right in a more professional way.