Flipping erected LED boards and land lease

We have 2 back to back 16mm LED billboards installed along a 2-lane highway (21,000 daily traffic) in PA. There are about 500,000 people in our county and we are 12 miles from a city of 60,000 and 18 miles from a city of 88,000. The local community, within about 5 miles of our signs is about 20,000. The boards are smaller at 7’X12’ although the speed limit is 45 mph and the signs are very close to the road. Just like the larger boards, a passing motorist can view and read about 2 ads each pass. They have been up since mid August and we have 6 spaces on each side, each displayed for 8 seconds. We have set the 4-week cost at $695 and right now have it about half full, although the average price for those 6 advertisers is around $500. It has been more difficult than we anticipated getting advertisers. The idea behind the smaller signs was to make this form of advertising affordable for smaller local businesses. At this time we are considering selling the location and are wondering should we use the formula I saw on a post here about flipping static signs to determine the value? That formula would go like this: $500x6x2x12x.6x10 or $432,000. The landowner’s take is 15% minus a monthly guaranteed rent of $500. If I then take about 60% of that the price I come up with about $260,000.I believe an experienced operator would have little difficulty filling these boards at the $695 rate so what would be a reasonable asking price. That would put the formula total at $600,480 and at 60% that would be an asking price of around $360,000. Any advice would be appreciated. Thanks.