Our agreement has overtime tiers beyond 18 hours. How do I price them?

Price the penalty tier as its own line. Model everything up to 18 hours in the Overtime AddOn, then add a Cost Item beside the position, name it for the clause it comes from, and put the agreed hourly figure in Fee. The tier then shows up in the budget under its own name, and it handles any tier your agreement defines.

Price the tier as its own line

Model everything up to 18 hours in the Overtime AddOn as normal.
Add a Cost Item beside the position and name it for the clause, for example "Penalty hours beyond 18".
Put the number of penalty hours in Amount, the days on which they occur in the day count, and the agreed hourly figure in Fee.
Leave the percentage AddOns of the original position off this line unless the agreement says the penalty is subject to them too.

A worked example

A gaffer is booked at 400 a day on an 8-hour day, so the hourly figure is 400 divided by 8, which is 50. The agreement pays hours beyond the eighteenth at 2.5 times the normal hour. You work that out yourself: 50 times 2.5 is 125.

Two days run into the penalty tier, three hours each. The penalty line reads Amount 3, two days, Fee 125: 3 times 2 times 125 is 750. The Overtime AddOn on the gaffer's own line keeps handling the hours up to 18.

Good to know

You work out the penalty hourly figure from the agreement and type it into Fee. The Overtime AddOn derives its own rate as Amount multiplied by Fee, divided by the end of the first bracket, and your line carries the figure the agreement names.
The bracket slider runs from three hours below the end of the standard day up to 18, in whole hours.
The extra line appears as its own row in any Offer Document table that lists individual Cost Items. A table built on chapter totals folds it in with everything else.