Startup valuation remains among the most contested problems in finance, owing to absent or negative near-term earnings, extreme uncertainty, and the circularity embedded in the dominant practitioner heuristic: peer-group multiples used to set entry price, followed by DCF reverseengineering to produce post-hoc justification. This paper extends the Risk-Adjusted Payback Period (RAPP) model of Baek (2026) to the startup context, introducing four principal contributions. First, we establish the Risk-Monotone Payback Horizon Principle (RMPHP): higher investment-stage risk should shorten the acceptable payback horizon T*, not extend it, because investors in earlier-stage ventures must demand faster fundamental validation before risk capital is considered justified. Second, we develop the Stage-Calibrated RAPP (SC-RAPP) framework comprising: (1) a three-phase EPS projection model accommodating pre-profitability trajectories (Burn, Ramp-Up, and Maturity phases); (2) a startup-adapted Discount Rate-Payback Period Mapping (DRPM) formula anchored to terminal-state parameters with explicit survival-probability adjustment; (3) a RAPP Fair Entry Price derived from projected EPS over the risk-calibrated horizon; and (4) a Reverse RAPP diagnostic that decomposes any peer-group valuation into an implied future EPS and business metrics target, converting the circular post-hoc DCF justification into an auditable, falsifiable business assumption. Three stylized case studies spanning Series A SaaS, Series B marketplace, and Pre-IPO platform illustrate the framework. Stage-calibrated T* reference tables and sector adjustment guidelines provide practitioners with direct implementation support. SC-RAPP offers a theoretically grounded alternative to the peercomps/DCF-reversal approach that is computationally transparent, communicable to nontechnical stakeholders, and free from the circularity and sentiment-inheritance pathologies of relative valuation.