A lot depends on yields... but here's some rule of thumbs to go by...

A very efficient operation will have an operating efficiency ratio of about 60-65%. So, 35-40% of the gross will go to cover family living costs (wages) and term debt service. Assuming that you are making the rest of your acreage pay for the equipment and personal labor, 35-40% of it can go for debt service on that ground. And that's a very efficient operation.

And a lot of farmers will argue until they're blue in the face that they're more efficient than that. They're wrong. I've seen way to much research that shows those numbers above. All the Farm Mgt research from the various states comes from actual income/expenses of those farms. Over time, that's what it's going to be.

Here's the link to illinois website showing actuals... http://www.farmdoc.uiuc.edu/manage/index.asp

That'll give you a good starting point.