I always thought the digital scale method was reasonable. As for the accuracy, I actually did check the accuracy of my digital scale, and it was almost spot-on to the actual weight applied.
There's a guy that's from Ohio that came to our modular club meeting with a home-brew pulling power measuring car. He was using a car with an embedded strain gauge and Bluetooth to transmit the results to a phone. We were having fun measuring pulling power for all our locomotives. I was happy to see that my Vision Line Big Boy topped the list for a single locomotive right at 7 pounds of pulling power, I guess a lot of weight and a big Pittman motor are a good combination.