Herman, surely depreciation is a notional (accounting) figure that is applied across the inventory ... and one which the esteemed Bean Counters are likely to fiddle about with over the typical lifetime of equipment anyway?
In other words, just pick a number (such as 7% pa), and run with it!
To my (traditional) mind, it is more important to keep a close watch on maintenance expenditure, and the point at which further repair work (for example) becomes no longer justifiable (unless special conditions over-ride).
Anyway, you can always add as many fields
as you like!