One way is to observe zero below a vertical deflector.
Then using a headcount ,,, like 321 321 321 and then read number below deflector.
You also might use thumper and use its time interval ,,,
One other thing is to use human reflexes to increase accuracy.
Like clucking with our tongue like a horse walking ,,, cluck cluck cluck cluck when taking rotor speed.
The crucial part is to be exact with same interval for each time you estimate.
So you get similar or same results.
Hey ,,, that's why roulette is a game of skills and not luck
Lets say you would like much action and pick the rotor timing that is very common ,,, if we assume you only will play one rotor speed when you build your playing model ,,, then you might discover that speed is not with the best scatter patterns.
So you might discover that less action can give you more precise results in the end.