The formula depends most on the player. I haven't spot direct formula like yours, but maybe there is one 😃 .
For example me, the formula is:
- If it is a nex tier building upgrade - accept
- if the BCP < 10 - accept
- if the BCP <= Pop gain - accept
- otherwise reject
My feeling is that on tier upgrade you get less BCP if you spent them before on previous upgrades. But this is only a feeling.
Also I have a little deficit of BCP with this rules. Good sign is that every day I upgrade about 10 times and most of the times I have less than 5 BCP in costs, so I don't notice them.
In the past I had second condition with "8" instead of "10", but then I got surplus of BCP and I found it pretty annoying (time effort for rejecting upgrades was pointless for me). So I switched to 10.
Also I have found that since I have less than 150-200 BCP in stock, I don't get 'blue loop zone' very often. I don't know if it is a rule or it is just a coincidence.