Overtime should only apply to my shoot days, not prep and wrap

To charge overtime on shoot days only, split the position into two Cost Items: one with prep and wrap and no Overtime, one with the shoot days and Overtime. Each half is priced on its own, so prep and wrap stay at their plain rate.

The split follows from how the AddOn works. On a Production Cost Item the Overtime AddOn charges Prep, Shoot and Wrap together at one set of brackets. The AddOn's sentence line names the day count it is charging, so a position with two prep days, three shoot days and one wrap day reads as six days.

Split the position

Keep one Cost Item for the prep and wrap days at their rate, with no Overtime AddOn on it.
Add a second Cost Item for the same position carrying the shoot days, and drag the Overtime AddOn onto that one.
Name both so the pairing is obvious, for example "Gaffer prep and wrap" and "Gaffer shoot days".
Put any percentage AddOn the position carries, such as employer contributions, on both halves, so the fringe covers the whole fee. Duplicating the first half with Duplicate Selection (⌘D) carries its AddOns with it.

A lighter alternative

If the difference is how long the prep days run rather than what they pay, keep one line. Switch the AddOn to Individual mode and record fewer hours for the block of days that is prep. Those days then reach no overtime bracket and are charged nothing extra.

The Days slider in Individual mode is a plain run of days, so lay the blocks out in the order the schedule runs, with prep at the front and wrap at the end.

A worked example

A position with 2 prep days, 3 shoot days and 1 wrap day, Fee 500, and an 8-hour standard day with a bracket to 12 hours at 50 percent. The hourly rate is 500 divided by 8, which is 62.50, so four overtime hours cost 4 times 62.50 times 1.5, or 375 a day.

On one line with Overtime, all six days can reach the bracket: up to 2,250.
Split, the shoot half carries the three days: up to 1,125. The prep and wrap half carries no overtime.

Good to know

Splitting the position doubles the rows in any table that lists individual lines. A document table built on chapter totals folds the two rows back together.
Simple and Individual are the Overtime AddOn's own modes. They are separate from the Unified and Individual modes of percentage AddOns.