I am a fan of spread sheets.  It is amazing how much a single Excel file can tell you about profit margins and how to save extra dollars when farming.  A few hours of crunching numbers told me that a tractor-plow configuration that I had in my mind was actually much to expensive because of some factors I had not given much thought to.  

One of the problems of spread sheets that I have found is finding accurate data to fill in the variables.  www.tractordata.com has lots of information about fuel use, drawbar HP, as well as many other tractor related information that I found was vital in finding accurate operating expenses.    If you are looking for a new or used tractor, combining the tools of www.tractorhouse.com and www.tractordata.com can help you make a good choice without having to call a million dealers and hunt for information. 

When using spread sheets, you need to decide how you are going to base your unit of cost / profit on.  For example, I use acres so I can easily find profit margins of individual fields and see how scale can save me money.  Another idea is to do it per cost per hour, or cost per day, etc.  But once you find your unit, stick to it for clarity. 

Also, it is important to make sure your formulas and numbers are written in a way that is readable and can be understood by others, such as a banker or a loan officer.  Having another person read it and make proofread the number will make sure the spread sheet is accurate.