Help in ring oscillator design

[saeedgreat]

hi. i created a 3 stage ring oscillator using 180 nm technology.my problem is when i try to compute the tphl from formula it doesn't match with what i got from analyses.
***ring oscillator*** .param Supply=2.5 .temp 70 .param WP=780nm fal=1 ris=1 .GLOBAL vdd Vdd vdd 0 DC='Supply' .ic v(in1)='Supply' **tp1** .measure tran tphl1 trig v(in1) val='Supply/2' rise='ris' + targ v(in2) val='Supply/2' fall='fal' .measure tran tplh1 trig v(in1) val='Supply/2' fall='fal' + targ v(in2) val='Supply/2' rise='ris' .measure tp1 param = '0.5*(tphl1+tplh1)' $calculate freq .measure freq1 param = '1/tp1' $calculate freq **tp2** .measure tran tphl2 trig v(in2) val='Supply/2' rise='ris' + targ v(in3) val='Supply/2' fall='fal' .measure tran tplh2 trig v(in2) val='Supply/2' fall='1' + targ v(in3) val='Supply/2' rise='2' .measure tp2 param = '0.5*(tphl2+tplh2)' $calculate freq .measure freq2 param = '1/tp2' $calculate freq **tp3** .measure tran tphl3 trig v(in3) val='Supply/2' rise='1' + targ v(in1) val='Supply/2' fall='1' .measure tran tplh3 trig v(in3) val='Supply/2' fall='ris' + targ v(in1) val='Supply/2' rise='fal' .measure tp3 param = '0.5*(tphl3+tplh3)' $calculate freq .measure freq3 param = '1/tp3' $calculate freq .measure tp param = 'tp1+tp2+tp3' $calculate freq .measure freq param = '1/6*tp' $calculate freq .subckt inverter in out WidthP=WP Mn0 out in 0 0 NMOS W=300nm L=250nm Mn1 out in vdd vdd PMOS W=WidthP L=250nm Cload out 0 1.5f .ends inverter x1 in1 in2 inverter x2 in2 in3 inverter x3 in3 in1 inverter .tran step=0.5p stop=4ns uic .lib "cmos250.lib" TT .op .END
this is my code and this is required formula also i have problem calculating the cload and c wire in it.