Plan
Two tools that answer different questions. Pit loss and overtaking threshold are fetched automatically for the circuit hosting the weekend.
Driver
One car, no traffic. Calculates the time loss for every conceivable pit lap and compound order from the library's degradation and the weekend's base pace. Use it on Friday–Saturday to see how wide the window is, whether one or two stops is fastest and what the hard tyre is worth – before the grid order is known.
The field
The entire grid is simulated lap by lap from the start: dirty air, the circuit's overtaking threshold, rivals who cover and undercut. The answer is finishing position, not race time. Use it once the grid is set to pick your stop lap against the cars you are actually racing – it is usually earlier than the Driver curve suggests.
Rule of thumb: Driver gives the time-optimal window, The field gives the position-optimal one. When they differ, it is the undercut threat or traffic that moves the stop. Each circuit's pit loss (green/VSC/SC) and overtaking threshold are listed for every track under Trivia → The circuits.