Towards the end of a run, the question arises whether it is worth continuing to purchase buildings. Even though the extra production is small as a fraction of the total, it is still extra production, so there is no reason not to get it.
UNLESS you have Chocolate Egg. This releases extra cookies equal to 5% of the cookies at bank at the time of purchase of the egg.
People have realised that it's generally a good idea to purchase the egg at the very end of the run, and before doing so to invoke the Earth Shatterer aura then sell all buildings, which will dump 85% of their purchase price back into the bank.
What if the end of the run is some time off though? Those cookies you have in the bank will bring you an extra 5% at the end. On the other hand, if you use them to buy buildings now, at the end you will get back 85% of what you paid for them, and you will also have extra in the bank from the increased production. Should you buy, or just keep them in the bank?
Let's do the analysis:
Number of cookies available in bank = C Time cost of buildings = T Fraction of cost recouped by selling = f. If Earth Shatterer is active, f is equal to 0.85. Estimated time till end of run = t
Buying buildings with those cookies will generate extra production dP = C/T By the end of the run this will generate extra cookies Ct/T
This is to be compared against the decreased bonus from Choc Egg. Selling back those extra buildings at the end only returns f * C, so the loss in cookies-at-bank is (1-f)*C, and the lost bonus is 0.05*(1-f)*C
I should therefore buy buildings only if t/T > 0.05 * (1-f) = 0.05 * 0.15 = 0.0075 with Earth Shatterer
The message here is pretty clear. When Choc egg is unlocked, estimate T and also t, the number of hours remaining for the run. I have an algebraic formula to estimate t, but otherwise just using the duration of previous runs as a guide. Once t becomes less than 0.0075T, which may be earlier than you think, stop buying buildings and just let those cookies pile up. (There may be an exception if you are aiming for a milestone such as 300 of some building which will give you an extra achievement and more milk.)