Yes, a degree wheel is what you say. I think you can get a magnetic one if you really want one but a print out stuck to a piece of card and some Blu-Tac is cheaper. Again what you describe is a clock gauge. Most are a straight movement but some have a lever arm. If you have a magnetic mount for yours, set it up to measure vertically from the valve spring retainer. Use the same position on each one. Remember that the rocker has a 1:1.5 ratio and to factor in your valve clearance. Figures are often very different due to cam wear. As an aside, eyeball along the valves, with the rockers removed for the best results, and see if any of the valve stems are higher than the rest. This can indicate valve recession into the head. It will be interesting to see your numbers. If the cam is worn it might be difficult to be sure of the cam type.