I think the fet spice models from the foundry are BSIM4 which uses binning. By using 'grep sky130_fd_pr__nfet_01v8__model <sky-pdk>/latest/cells/nfet_01v8/sky130_fd_pr__nfet_01v8__tt.pm3.spice -A3' one can see the spice definitions for the bins. I think specifying only W and L might be good enough as long as RC extraction tools generate devices with nf=1 and mult=1 (I believe this is the usual case). ngspice will be picking up the correct spice paramters of bin for the simulation. According to the BSIM4 manual when PS is given it updates Pseff as