I always found it very tiring to calculate the distances for cross-sections upstream and downstream of culverts, so I have developed a simple Excel-based calculator.
All you have to do is enter your culvert station and other parameters and it will tell you the location for the upstream and downstream calculations.
Unfortunately, I cant upload an excel file here, so I have saved it at the following location:
http://deccantrap.googlepages.com/culvertcalculator.xls
Please feel free to post your comments, or make improvements to the file.
Thanks!