C150 instructor pilot reported fuel starvation resulted in engine shut down and an off airport landing.

Narrative

We were preparing to depart on a local area training flight; the student was performing a supervised pre-flight inspection and advised that we had 10 gallons of fuel. I 'back of the envelope' estimated fuel burn for the C150 at 5 gallons per hour. I intended to be back to the airport within 1.5 hours allowing for a 30 minute reserve. While 2 miles out from the airport on approach; the engine quit due to fuel exhaustion; flight time at this point was 1:21. I made an uneventful landing off airport landing; there was no damage to the aircraft or any injuries. A visual inspection of the fuel tanks revealed no fuel in the left tank; and a small amount in the right tank. After adding fuel; the engine started an ran without issue.The POH for our actual power setting / altitude / temperature specifies actual burn should be 4.3 GPH. The dipstick being used was a home-made paint stick with sharpie markings; I had not personally verified the accuracy of the stick. I also did not personally dip the tanks after the student. This plane does not have a flow meter; fuel totalizer; or EGT; we leaned 'generally' by just pulling the mixture out an inch-'ish'; which could have caused over burn.Of course; the easiest solution would have been to simply add fuel before departing; rather than relying on rough estimates and a 30 minute reserve.