The next time you put on a new Distributor cap save the old one. I've kept two old caps (55-56 style and 57 style) and cut them in half. I've then painted eight (8) white dots around the cut portion corresponding with position of the rotor when each piston is at TDC (Top Dead Center). To make it, carefully cut the top off. On both of them I had the cap crack. Clear silicone sealant was a ready cure to put it them back together. After locating cylinder number 1 TDC the rest are easily spotted by dividing the cap into 1/4 and then 1/4 again centered between the first set of four. The photos below will make more sense of this.
When adjusting the valves remove your distributor cap and snap this modified one in place. Jog the engine until the rotor points to the paint mark for Cylinder #1. Now adjust the valves on Cylinder #1. Carefully jog the engine until the rotor points to the next paint dot and adjust the valves on Cylinder 5. Proceed through all 8 Cylinders in firing order number: 1-5-4-8-6-3-7-2