how do I calculate the percent of gross income on line 7 of schedule C that is from the business use of my home?

That percent is based on your particular circumstance for your business.  

 

For example, if all your work is done in your home office, then your percent of gross income from business use of your home would be 100%.  

 

But, if you also do work in other locations other than your home office, then you have to determine how much work is done in the other locations as compared to the home office and choose the percent accordingly.