making Tphl and Tplh equal
There is no need for guessing, but you will need to look at the transistors data-sheet for \$I_DS,Sat \$ per width, and you should understand what happens when you stack transistors.

Alternatively, you can set the \$ \dfrac{G_m}{C} \$ for the NMOS and the PMOS to be the same.

I will note several odd things with your netlist:


you have all the same size NMOS's and all the same size PMOS's in your NAND which.
your width ratios are odd too, PMOS/NMOS ratios are more typically 2.2X you have 2.0 X
all you PMOS are very long 4X longer than the NMOS.