Some days ran long and some did not. Can I charge overtime per day?

When two days of a five-day shoot ran long, overtime belongs on those two days and nowhere else. The Overtime AddOn does this in Individual mode: you split the day count into blocks and say how many hours each block ran, and only a block that reaches an overtime bracket pays for it.

The mode button sits at the start of the AddOn's sentence line and reads either Simple or Individual.

Which mode to use

Simple assumes every day on the line runs to the top of every bracket you defined. It suits a worst case, a contractual guarantee, or a deal memo that prices a long day flat across the whole shoot.

Individual follows the schedule you actually expect. Use it as soon as the days differ.

Set the days out individually

Expand the Overtime AddOn and click the mode button until it reads Individual.
A second section appears with a slider labelled Days.
Split the day count into blocks the way you split the hour brackets: hover between handles, click the +, then drag the handles to set where each block ends.
Enter the hours worked for each block.

A worked example

One person, five days, Fee 800. The standard day ends at 8 hours and one overtime bracket runs to 12 hours at 50 percent. The hourly rate is 800 divided by 8, which is 100, so four overtime hours cost 4 times 100 times 1.5, or 600 on a long day.

Simple charges those four hours on all five days: 3,000.
Individual, with a two-day block at 12 hours and a three-day block at 8 hours, charges the two long days only: 1,200.

Good to know

Simple and Individual are the Overtime AddOn's own two modes. Percentage AddOns such as handling have a different pair, Unified and Individual, which choose one rate for everything or a separate rate for each. The two pairs are independent of each other.
The Days slider runs as far as the Cost Item's own day count, so the blocks always sit inside the days the line carries. When the schedule grows, raise the day count and extend the blocks to match.
Hours are whole numbers, and brackets run up to 18 hours.
On a Production Cost Item, Overtime works on Prep, Shoot and Wrap together. Lay the blocks out in the order the schedule runs, so a block at the front of the timeline is prep.