The formula for spin drift is pretty easy, if you know your stability factor (SG).
SG is more difficult to calculate, but fortunately there are online calculators that can do this, such as the one on the Berger site: http://www.bergerbullets.com/twist-rate-calculator/
So, after you get your bullets SG, plug it into this formula for spin drift:
For example, if your SG is 1.8, and the tof to 1000 yards is 1.6 seconds, the spin drift would be:
SD = 1.25*(1.8 + 1.2)*1.6^1.83
SD = 8.9 inches.
-Bryan
BTW, the screenshot above is from Chapter 6 of: "Applied Ballistics for Long Range Shooting", if you're interested in more detail.