Electronic – making Tphl and Tplh equal

Let's say we have the following netlist

* Standard cell circuit

.global VDD VSS

.prot
    .lib '.\reference_models.lib' TT
.unprot

***********************************************************
.SUBCKT ZP_NAND2 QN A B 
M1I1 N1N9 B VSS VSS N W=WN L=LN 
M1I2 QN A N1N9 VSS N W=WN L=LN 
M1I3 QN A VDD VDD P W=WP L=LP 
M1I4 QN B VDD VDD P W=WP L=LP 
.ENDS 

.SUBCKT ZP_INV QN A 
M1I1 QN A VDD VDD P W=WP L=LP 
M1I2 QN A VSS VSS N W=WN L=LN 
.ENDS 

.SUBCKT AN2D1  Z A1 A2 
X1I8 N1N1 A1 A2 ZP_NAND2 wp=1.62um lp=0.35um wn=1.07um ln=0.09um 
X1I9 Z N1N1 ZP_INV wp=2.70um lp=0.35um wn=1.35um ln=0.09um 

c1   Z   VSS 0.018pf
c2   A1  VSS 0.009pf
c3   A2  VSS 0.009pf
.ENDS 
**************************************************************
X1  Z A1 A2 AN2D1
*************************

.param vdd = 1.8
.param vss =0

Vvdd VDD 0 VDD
Vvss VSS 0 VSS

Vin1 A1 0 pulse (vss vdd 2ns     200ps     200ps     3.8ns       8ns)
Vin2 A2 0 pulse (vdd vss 4ns     200ps     200ps     2ns         8ns)

.TRAN 1p 20n

.measure tran pow_avg_X1 avg p(X1)
*.measure tran pow_avg_mn avg p(X1.mn)
*.measure tran pow_avg_mp avg p(X1.mp)
*.measure tran total_power param='(pow_avg_mn+pow_avg_mp)'

.measure tran I_integ integ I(Vvdd)  
.measure tran P_Iinteg param='I_integ*vdd/20n'

****************************************************
.meas tran TPHL trig v(A2) val='0.9' TD=0
+   fall=1 targ v(z) val='0.9' fall=1
.meas tran TPLH trig v(z) val='0.9' TD=0
+   rise=1 targ v(A1) val='0.9' rise=1
********************************************************



.end

When we run it in hspice, we get the following result for Tplh and Tphl

tphl=  1.1032E-10
tplh= -6.6974E-11

How do we make them equal(not necessarily 100% equal,, but lets give them a maximum difference of only 3%)? The only thing that we can vary is the transistor's width (we need to make the 'L' fixed to 90 nm). Aside from randomly guessing the value of 'W' to make them equal, are there any methodologies out there to follow?

Thanks