A depreciation method should reflect how the asset's service potential is used. Double-declining balance applies twice the straight-line rate to beginning book value. Salvage is not subtracted when calculating that rate, but depreciation is capped so ending book value never falls below salvage. The remaining depreciable amount is taken at the end of useful life under the convention used here.
Units of production allocates cost according to output. Divide cost less salvage by expected lifetime units, then multiply by actual annual units. An idle year produces no depreciation under that method. Sum-of-years-digits and 150% declining balance provide other allocation patterns. Compare methods using the same cost and estimates; an accelerated expense does not change the cash paid for the asset.