Long story short, for ISC ratings above 1300, multiply by .92 to get an approximate NSA rating.
There's a lot more magic voodoo math mumbo jumbo to be done, but basically:
ISC spans from 0 - 2200 with the best legit player rated 2045 (Weinstein NSA 1897).
NSA spans from 200ish (a rare case, with 500 floor) - 2100 (these days) with the highest rated player at 2047 (Richards). (US is Cappelletto at 2009.)
Others I've tried before arriving here: subtract 500 from ISC and multiply by 1.25, but that doesn't seem to be nearly as right at the top of the ladder because all the best players are bunched together and constantly switching places. I used to say a good rule of thumb is to simply subtract 200 (again, for ratings above 1300). You'd have to get a whole lot more data to get a more acurate conclusion, especially since ISC ratings are way more volatile than NSA ones.
In a nutshell, I'd guess an okay approximation is:
But imagine that .92 actually squeezing closer and closer to 1 the closer you get to 1300 (by what factor I don't know -- the multiplier should probably be derived as a function of your rating itself, much like in that awful gameshow with the briefcases of money that Shall Not Be Named, in that the multiplier used is constantly changing there too, and a function of what's left -- but that's even hairier).
Yes, I'm bored.