Even if 20% per year depreciation is accepted, it should be applied to the residual value at the end of each year. (Compound interest rather than simple interest.) After 2 years the residual value should be 64% of the original price, rather than 60%; after three years 51.2% rather than 40%.

Of course the 20% base rate is ridiculously high. I have one suitcase that has supported my travels at Silver-maintaining levels for over 20 years and is capable for going on for a few years yet.