Digital Lease/Permit Valuation

First off I want to thank Frank for all of his support on this forum and all the information that he has made availabe to us through his books/cd’s/courses. Using his techniques I have currently flipped 3 lease/permit’s for double sided bulletins. However I see alot of people converting their signs into digital and would like to try my hand at flipping a digital location. I have a site in mind and have seen some other digital boards in this area renting for $1500 per add and usually run about 6 adds on each side. My issue is I do not know how to accurately valuate the lease/permit for this. For my static bulletins I have been using a estimated net profit with a 20% cap rate. Frank, can you help me with putting a value on the digital lease/permit?