Here's how I set the 1680 I used to run, I can't remember the numbers and they wouldn't help you much anyway, since the rotor is bigger in a 1680 so it runs a lot slower.  Anyway, set your concave clearance somewhere in the middle of the factory suggested settings for corn.  At a combine clinic I went to this summer, the Case rep said the clearance should be basically the size of an average bare cob.  I rarely if ever adjusted my concave clearance once I got it set for the season.  You can almost always do what you need to do with the rotor speed once you get the clearance in the ballpark.  Once you have the concave set, set your chaffer and sieve to give you a good sample without too much going into tailings.  I know quite a few guys that run the bottom sieve wide open in corn and just do their cleaning with the top chaffer.  Anyway, to set your rotor speed, start somewhere in the middle of the range in the book.  If your corn is 16% or less start towards the bottom.  Run a little ways, stop things, get out and check how the cobs look.  If everything is coming off, go ahead and slow it down a little more.  Keep repeating until when you get off to check, you find some cobs with a few kernels of corn on them.  Then speed your rotor up 50-100 rpms, whatever it takes to get back to cleaning the cobs off.  This way you'll be getting the corn off at the slowest speed, which should allow for less damage and still keep your throughput high.  Be aware that different hybrids at the same moisture can require significantly different rotor speeds to do a good job.  Last year I switched fields and the hybrid in the second field was a little dryer, but didn't come off the cob nearly as easy.  Also, I did do quite a bit of 15% corn last year at the lowest rotor speed I could get, so don't be surprised if you run into that too. 

Hope this helps,

Mike