Not all capital spending is the same. Some of it just keeps the existing business running (replacing worn-out equipment, maintaining facilities); the rest funds new growth (new factories, new stores, new product lines).
Why it matters for owner earnings. Warren Buffett's original definition of owner earnings only subtracts the maintenance portion of capex — spending required to preserve the company's current competitive position, not spending made by choice to grow. Subtracting all capex would understate a growing company's true earnings power; subtracting none would overstate it by ignoring the real cost of staying competitive.
Default: 80% of total capex. Companies rarely break this split out explicitly, so 80% is a reasonable, slightly conservative estimate across most businesses. If you know a company is in a heavy growth-investment phase (a lot of its capex is genuinely new stores or new capacity, not replacement), you might lower this — but do so deliberately, not by default.