What is the recommended standoff size to help with compression of the ball on the flywheel?
That varies depending on your flywheel configuration. I’d recommend trying different lengths for different compressions. You’ll also want to try putting something compressible (like foam or grip-mat) on the backplate so that something in the system can squish.
How do you test it to see if you are getting the best compression
When the ball goes the furthest.
It’s hard to recommend a standoff size as @sazrocks mentioned, but I would recommend making sure that the back plate is very consistent. If the compression is greater in one area than another, it will severely hurt the shot. You want to find a compression where the path for the ball is consistent in compression and firm, but no spots that are so tight that the ball gets stuck. It is often helpful to hand test the compression because that will give you a better feel of how good the compression is. You may end up getting a great shot, but then by hand testing the consistency of the hood, you might find that it could be even better.
So I was thinking two 18:1 V5 motors with the 25:1 gear ratio, or should I use 6:1 V5 motors