We investigate a new metric slope Root Mean Square (RMS) for characterizing the performance and for large telescope such as TMT tertiary mirror. The metric can multiply the errors which derived from the low order aberration, such as Zernike polynomials and other frequency composition. It represents the efficiency lose due to mid and high-spatial frequency components in frequency domain. The metric also properly counts for the optic error of the mirror itself such as the deviations from a perfect surface and metrology measurement errors. We also numerically confirm this feature for the TMT M3. We hope the work of this paper will be useful for the metrology of TMT.