The curd timer

A simple model says that bacteria in warm milk divide again and again, and that the pH of the milk falls as their number rises. You will write the model and use it to plan a batch of curd.

The idea. The count of bacteria is $\text{start} \times 2^{\text{divisions}}$ up to a ceiling of $1000$ million per mL. The pH is $6.7 - 2.2 \times \text{count}/1000$. The milk sets when the pH is $4.6$ or lower.

Your tasks.

1. Finish bacteria. It gives the count in million per mL after hours.
2. Finish ph. It gives the pH of the milk.
3. Finish hours_to_set. It gives the hours until the pH reaches $4.6$.
4. Set least to the smallest starter in the list for which the milk sets within $5.5$ hours at a division time of $40$ minutes.