Owner earnings is a valuation method detailed by Warren Buffett in Berkshire Hathaway's annual report in 1986.
Average annual capital expenditures is, as Buffett mentions, an estimate.
This number may be separated into maintenance capital expenditure (what's required to keep the business operating at current levels) and growth capital expenditure (what's required to grow the business).
The most straightforward way to calculate maintenance capital expenditure is to simply use depreciation, amortization, and depletion.
One way, as proposed by Bruce Greenwald, to approximate maintenance capital expenditure, for use in an owner earnings calculation is to use a ratio of capital expenditure to sales over multiple past years.