Hands are unique, it is not just a measurement.
I believe though that index to pinky is more representative of the hand's ability to spread across frets.
Each of the fingers will be longer/shorter, perhaps have an injury, the tips will be quite fat or slender etc
One can do quite a lot to improve with technique, but if after playing for quite a while on a 650 you still struggle it might be time to look at exactly what the issue is and either a shorter scale, or tweaking the set-up you have.
Other factors to consider
- E-E string spacing at the nut, not just the width of the guitar's neck (can be a very easy fix such as having the nut removed and a new one put in with a different string spacing put in) a luthier can do that even 1 mm less can be great
I think in your case that actually might go a long way in helping, given what you described.
- with either narrower neck or narrower E-E, your precision has to be better but might find so much easier to access upper strings if fingers are short
My current guitar is 640/52 with 41mm E-E which is narrower than standard, It is set-up that way as I have a very short pinky, works great
- consider the action, is it too high? often factory guitars do that to ensure they don't buzz, which would make the playability a bit harder
- depending on where you, yes do try 640's, try 630's but when you do, bring a good tape with you and measure the string spacing at nut, bridge, action, neck width and so forth and then you can see what elements of the particular guitar work for you