My oscillation frequency is 1.3MHz, so I set "tones=1.3e6," clk_out is the output node of the oscillator and 1.25 is a half of 2.5 of Vdd.
But, it takes me a lot of time on running simulation all the night.
Use shooting newton analysis prior to phase noise extraction instead of harmonic balance. Your ring oscillator is working at a frequency much lower than the highest significant harmonic.