Mining ROI is usually estimated as annual operating profit divided by hardware cost — a useful starting point, but only ever an estimate, since it holds price, difficulty and uptime constant for a year that won't actually stay constant.
The real number is whatever your wallet actually receives. Scrypt Wallet tracks real, current balances and prices across every coin you mine, side by side, so your actual return is always one glance away rather than a projection you have to keep re-running.
Create a free account to track your real mining ROI, non-custodially, across every coin you receive.