This is just a quick and dirty version but should help you get started. It is set up for a 20 horse field but can be adjusted for lower according the instructions on the sheet.
Have a look and hit me up via Twitter (@loomsboldly) if you have any questions or problems.
And here is an improved version of the sheet courtesy of Ryan Coli (@C_ryran):
The Unbearable Lightness of Betting 
This is perfect, just what I was looking for! Thanks for posting it, Nathan
Excellent, happy to help! And thanks for listening to the #DRFPOD !