Final answer:
The missing part in the units-of-production depreciation formula is 'current-year activity or production', used to allocate depreciation based on the asset's actual use or production level.
Step-by-step explanation:
The formula to calculate the depreciation for the units-of-production method also known as activity-based depreciation, is ((cost - residual value) / total estimated production) x current-year activity or production. This formula allows businesses to allocate depreciation based on the actual use or production level of an asset, which can provide a more accurate reflection of its consumption over time. Depreciation is an accounting practice which spreads out the loss of value of an asset over the asset's useful life, and is factored into a firm’s accounting profit, reducing total revenues to arrive at net income. The other components mentioned in the question, such as total activity or production, gross profit percentage, and historical cost, are not used in this particular depreciation calculation.