you dont need as much bpt with lower land though

unless your talking about rebuilding a lot, via gov switches, ab/br damage or land trading

and one of the big factors with bpt is if you dont get as much land you waste turns, if you get too much land you waste turns, this could minimise both of those factors

the other way i would go though is:

=(csites/4+5)^power*govmod

potentially even combining both ideas

anyway depending on what you wanted to achieve id suggest 1.1 to 1.2 for the exponent

perhaps 1.1 with a 40k land mod as well