I just did a little playin' around with the calculator (always love that) with the specs you've given. I used 47.5 as your tire diameter and find your roll-out is 64.88mm (almost 65). Now it's easy to look and think "63mm...64.88mm...that's pretty close" but the reality is its darn nearly 3% high. Even figuring 47mm diameter you're at 64.2mm (2%). Your 40 tooth pinion will your 63mm roll-out figure when your tires are just over 46mm in diameter.

If you've got pinions in a range of 38-43 you can hit 63mm over the entire tire range from 48mm down to about 43.5mm (about the minimum I can run and keep 3.5-4mm ride height). If you added a 37 and a 44 you could experiment with some other targets, higher and lower, off either end of your tire diameter life. This assumes, of course, you stay with a 92 spur.

There are different opinions on this, and I'd CERTAINLY listen to what OD had to say on the matter. For club races I'll run tires at 48-48.5 new and run them down to 45 or so. The thing to keep in mind is that tires are REALLY easy to "chunk" at 47-48mm. I've gotten to where that doesn't happen to me very often, but there are no "savings" to be had running larger diameter tires if you get a hunk taken out of one.

Any tires that end up at 45-46mm go into a baggie and are saved for my next "bigger" race of which I hit 3 or so per year. Racing about every other weekend Oct-Mar I'm usually able to take 2 sets of tires that are run to that diameter to any of those races, and if I needed more I've got a tire truer