Is there a max benefit of 50% of land build as enterprise zones where as over 50% of your land build as such does not increase your PCI. I thought it was in the game info at one time but I can't find it now.

It benefits over 50%, but because your tax income is essentially your (1 + Ent built%) * (1 + Res built%) multiplied together, you want to make sure both Ent and Res and built in near equal amounts.

The largest volume a rectangle can be (tax income), given the same perimeter length (acres), is essentially a square (equal res/bus), ignoring the effects of population food upkeep.

I've been running my theo's at about 55res/45ent