[formerkrogemp]

Nah, we start with market value when new, a representative rate or method of depreciation, and a residual value. You can take a straight line of depreciation, double declining, etc. Residual value is usually a ballpark figure with the actual closing disposal value representing a loss or gain upon disposal. You're overcomplicating it taking in too many parameters to be a useful rule of thumb or guideline for those concerned with the numbers. As you're pointing out however, the relevant parameters do vary from person to person. Anywho, personal finance isn't really bookkeeping for a business.