On my vibrating shuttle machines, length doesn't seem to be real crucial as long as it's long enough & the distance down to the eye is close. Probably 'cause there's nothing underneath it to hit. I've even used Boye 4s on a couple of these....& they're nearly 1/8" longer than a 20x1, Boye 21, or 14 but the eye is really close to the correct distance.
But on a machine where there IS something for the needle to hit if it's a bit too long, I always turn the handwheel gently to check first. I'm sure you'd all do the same...
Here's another chart cross-referencing needles & machines.
http://www.ismacs.net/needle_and_sha...eedle-list.pdf