Here is a great tool that lets you calculate the carbon footprint of your Taxi/PHV.
When calculating the carbon footprint of your operation, you also need to calculate the carbon footprint of every other part of your business, such as the carbon footprint of your office, or if you work from home, a proportionate amount of your domestic footprint applicable to the running of your business.
Warning: Make sure when choosing an Offsetting company, that they are genuine, and that they really will plant the tress etc. Use a company such as Ecologi