Factors that affect the percentage of shade needed are local climate and crop needs. The most universal and general way to estimate local climate is with the USDA plant hardiness zones. This is based on averages and may not be reflective of your actual needs. Generally, other growers in your area may be the best source of knowledge as to which shade to use for your selected crop. 

To estimate the needed shade, one will need to know the ideal temps and light conditions for their plant, and also what kind of temperature and light change they might expect from their applied shade.  For example, if you planted a leafy green like lettuce or kale these plants can thrive in cooler temperatures.  So you might want a 50% shade cloth to drop the temperature of the tunnel approximately 10-15 degrees.  On the other hand, if you planted tomatoes or peppers you might only want 30% to drop the temperature approximately 5-10 degrees since these plants love the heat.

Ultimately, you will need to decide what works for you and your situation since not all plants, gardeners, farmers or locations are the same.