The formula you're using only determines MAX PCI not CURRENT PCI. But the modifier is correctly changed from 90k to 18k. You have that right. Current PCI is I think this:

=Round(If(MaxPCI>=CurrentPCI,MIN(pciA,-pciC),-MIN(pciB,MAX(-pciA,pciC))),2)
pciA =(1+0.03*(1-Taxrate))*(1-0.5*PopGrowth/NEWPop)*CurrentPCI-CurrentPCI
pciB =(0.035+0.3*Taxrate)*CurrentPCI
pciC =CurrentPCI-MaxPCI

PCI can decrease even if Max PCI is greater than or equal to CurrentPCI. This anomaly occurs when there's rapid population growth. At least....that's what I have written down in a random notepad and I get the feeling that's not all the information you need.