In the hard disk drive (HDD), the Wallace based in-situ flying height (FH) test is widely used for FH measurement since the application of thermal FH control (TFC). The Wallace equation shows the spacing loss of signal amplitude at a particular frequency or wavelength. It has a very simple expression for longitudinal recording and becomes more complex in perpendicular recording with soft magnetic underlayer. In the FH measurement, the signal amplitude at testing frequency has to be in good quality in order to achieve high precision FH measurement. In this work, an integration of power spectral density-based version of the Wallace spacing loss equation is developed. The use of the latter equation enables in-situ FH measurement via the monitoring of relaxed quality (multitone) readback signals over a band of frequency. The new method is able to test the FH change at high precision on AC erased track.