Yield maps and experience.   
 
Is 150 the max you ever see on your monitor or are you referring to field avgs?   As far as terrace channels we only see issues on new ones so if they aren't draining right get that fixed.   If you are the combine driver, you know what areas will yield so i'd figure max yield, add a couple thousand plants and set it there.  1 - 1.1 bu per 100 harvested plants, add 10% for planting and go.  Then you can adjust over the years as you gain experience.

I'm not sure i've seen much advantage to it myself, if it rains we grow really good corn, if it doesn't, we really struggle.   You won't see seed savings i bet, you could see some yield advantages but not always.