I found some of the guidance in the following documents useful in developing my own method to do this. You can obtain a copy of it in PDF format by searching the www.
Market Segment Specialization Program, Carwash Industry, Traning 3147-117, TPDS No. 853821L, Appendix B Indirect Method - Water Consumption Analysis Format and Appendix C Indirect Method - Soap/Chemical Consumption Analysis Format. This document is produced by the Department of Treasury, Internal Revenue Service.
The ICA also has some pertinent information that you may find useful for developing a water consumption - sale volume model. You could try searching for these on the www and/or try looking on
www.carwash.org
1) ICA Self-Service Carwash Saves Water brochure. I got my copy from Townco Equipment out of Austell, Ga.
2) Report on Water Conservation in the Professional Carwash Industry by Chris Brown, published by ICA, March 2000
Hope this helps.