Regression Analysis, R Squared, Pseudo R Squared, Adjusted R Squared, Deviance.
residual errors (RSS), the worst that you can do is the Mean Model. But the sum of squares of residual errors of the Mean Model is simply TSS, i.e. for the Mean Model, RSS = TSS. Hence for OLS linear regression models, RSS ≤ TSS. Since R² =1 — RSS/TSS, in the case of a perfect fit, RSS=0 and R² =1.