Summer Shade for Herbs

I hope you can advise me on selecting shade cloth for my new hoophouse. I grow potted herbs for a farm gate business and I want to keep them healthy over the summer months. I was wondering what percentage of shade cloth is suitable to keep them growing but not drying out too much?

The first priority is adequate ventilation. Ideally, herbs like as much sun as possible, so if you can enhance the ventilation as much as possible, you don't need as much shade to combat temperature build up in the summer.
For herbs that normally require full sun, you could try cloth that gives 20% shade. Anything more would compromise the ability of the plants to withstand sun and wind burning when customers put your plants out in the garden. Spraying shading compound on your plastic cover is also effective.