Joe, I'll echo what you said and add a little bit.
My fretting hand span is but 8"! Though it is index to pinky that really matters; still, I bet it correlates highly with the thumb to pinky.
One word of caution in going for shorter scale guitars (and by shorter I don't mean just going to 640mm or 630) - when you get down to 615, 600, or 580mm and thereabouts, you may notice significant differences in sound (even my tin ear does). The bodies of these guitars are smaller, and I believe that is a major factor. As is generally understood, going with higher tension strings helps to counteract the reduced tension from going to a shorter scale. But I think the body size plays a big role in the sound, and most shorter-scale guitars are smaller in body too.
If you are like me, your ears may not appreciate the smaller scale guitar (in standard tuning) as much as your hands do! To keep the sound pleasing to your ears, you may need to undertake quite a bit of experimentation with strings. This can get a bit pricey, but ask around for recommendations and perhaps save yourself some unfruitful efforts. Alternatively, don't go too small. Find your right size (I am relying on Prof. Kopfstein-Penk's book The Healthy Guitar for this) and yours is a borderline case, choose the larger one. For me, the closer to a full-size 650/52 guitar, the more the sound will be pleasing in standard tuning. If you require a much smaller guitar, say below 580mm, you will probably either need to tune it higher (say to G) or you'll need to really research high-tension strings, probably non-nylon ones, to get a good sound in standard tuning.
As an example, I am now trying out a 615mm scale guitar with high-tension strings. In terms of fret spacing, it's a great relief. In terms of string spacing, it's OK (at 48mm nut) and I will adjust. I sometimes miss the 52mm nut standard, but I also know there are vertical & diagonal reaches that are much easier now too. So the narrower neck has its pros and cons. In terms of sound, it's pretty good, not too far off my full-scale, but I can tell a difference if I really pay attention, and I do miss the richness of the full-scale 650/52 at times. You pays your money, you takes your choice, as they say.
Just to offer a contrast, though - I also recently acquired a Yamaha Guitalele, tuned A-A. The frets are REALLY close together, and that can be a bit of a challenge - but as far as sound, well, it's tuned A-A, so you don't expect it to sound like a CG. It sounds like it's own unique lovely thing, so the difference in sound to a full scale CG is not bothersome. Plus, since it's tuned higher, it doesn't sound bad in absolute terms either. That said, I have read of someone who tuned it E-E by shifting the strings over one place, detuning the C to B, and adding a low E. I wonder if they kept it that way.
When discussing short-scale guitars, Rob Mackillop once replied to a comment on his site that he would not recommend going below 600mm for standard tuning. In general, I can see why - it's hard (but not impossible) to get a good sound out of them. Either tune up or be prepared to experiment with a wide variety of high-tension strings.