That is about where we are at except we do not go that low because we don't have any areas that will yield that low as long as we get the moisture. You might want to check out the Pioneer web site as they have done quite a bit of testing on this subject. Basically they say areas that consistantly yield below 110-120 need about 22k and everything above that needs to be in the 28-32 range as the higher yielding areas pretty much follow the same yield curve no matter what the yield as long as it is above the 110 or 120 level. I may  not have that exactly right so be sure to check it out on their site yourself - been a while since I read it.

John