You also didn't factor in the destruction of indies and constructing main buildings that most people do before stocking if not playing an indy.

But anyway, one turn spent today is a lot less than 1 turn spent tomorrow. CS optimality around minimizing turns is assuming that all turns are of equal worth, which they are not. Cashing 5m/turn today versus cashing 35m/turn in 20 days proves the point. The optimal bpt forumla posted by Xin is what is an 'estimate' under his set of assumptions. (see opportunity cost, etc...)

But when optimizing BPT you have to ask yourself: what are my other options? For example, exploring vs. cashing vs. teching vs. building BPT.

Edit: You also didn't factor in getting hit and retaling. The total land you build on and the total land you finish with are typically different :p

Last set I only went to 100BPT on 60k acres, not to mention I probably built on close to 70k throughout the set. The reason being is that I mistakingly assumed I wouldn't get past 50k and that exploring for 1 turn would yield greater benefit.
This game is way too complicated to play perfectly :) It's all theory-crafting.