Flipping Leases

I am flipping leases/permits and I have a company that is offering a small up front buyout amout but offering a percentage of the gross revenue. The intial buyout will cover the permits and make a small profit but getting a percentage of revenue could potentially be huge. Has anyone else done a deal like this and how much of a percentage is fair and for how long of a time. I am thinking asking for 10% for the life of the ground lease. The ground lease is set for 10% already so this would bring the billboard company to pay 20% of the gross revenue which seems fair for everyone. Any thoughts or experiance are appreciated.