Ac analysis is the best method for PSRR
give ac source
vvpwr vpwr 0 VDC AC 1
use .ac statement
measure or view AC magnitude of output voltage.
It will give information of PSRR at different frequency
in the ac analysis sweep the ac frequency from dc to 2nd harmonic of the logic core frequency ( The ac source is applied in series with the dc power supply)
Measure the ac gain (in dB) from the ac source to the o/p.
The graph thus obtained (gain vs freq.) is your psrr at various frequencies