Calculating variable depreciation values

I am trying to make a calculator which will automatically calculate the depreciation values of vehicles based on age/mileage/purchase price.

I have been given the following figures to work with:

10% depreciation when registered
20% after 6 months
35% after 12 months
50% after 24 months
65% after 36 months

This is based on a fixed annual mileage of 6,000 miles per year. For every extra 1,000 miles depreciation is increased by 2% on purchase price.


So what I want to have is a kind of calculator where the user will input the purchase price, age (in months) and mileage, and it will calculate the depreciation value based on the conditions as above, e.g.
Value = 10,000
Mileage = 5,000
Age = 9 months
Depreciation value (output) = 7250

Please could someone give me a few tips on the best way to do this.

Thanks

The general idea is not too hard, but please explain your example calculation. It is not obvious how you arrived at $7,250 based on the situation you presented. Are the mileage and age inputs calculated ratably - i.e., is 9 months calculated at the average of 6 and 12 months (20% + 35%/2)?