You can calculate it off the tires on the tractor already, or at least get close enough for ag tires... OR just go with the size you have now, which is the factory offering in ag, turf, and industrial according to TractorData.com (The factory doesn't change ratios depending on your tire selection...)
If you can find larger tires and want them, just be sure to go the same percentage bigger front and rear.
If you're bad at maths, measure the height of the current front or rear tire as best you can. Find your preferred larger size for that tire, and divide the current tire ht by the ht of the larger replacement tire. That gives you a number like 1.10, which would mean your new tire is 10% larger than old. Now go find a tire for the other end of the tractor that's also 10% larger than the stock tire for that end. (Multiply the measured height of the current tire by 1.10, find a tire that height)
Make sense?
FYI, it appears that on my tractor, the ratio equates to the front and rear tires covering the same ground with the industrial tires, but the ag tires for my particular tractor did provide about a 5% lead on the front axle by the height numbers. I assume the tread on larger AG tires squirms a bit more, so forgives the lead going straight, and the slight lead makes turning easier... ?? By that example, I'd not worry if you wanted oversize ag tires, and the fronts were up to 5% larger than "ideal" numbers suggest. At worst, you'd get faster wear on hard surfaces in 4wd, but why would you be operating like that, especially with ag tires??
