Depreciation for commission earner 2015

As a commission earner, calculation depreciation. I use 2 vehicles for both personal and business. Would I calculate depreciation as price of vehicle / 5?
or would I factor % of business?

Do you keep a logbook showing the difference between business and personal travel?

Yes. Car 1 is used 65% travel business
Car 2 is 40% travel business

Then you would take the cost of each vehicle, multiply them by the months over 12 that you used the car then divide by 5 and then apportion that based on the percentages above.