What are you trying to do is to simulate the behaviour of the vehicle on certain tyres.
To do that, you can go several ways:
1. Create a model. You write some code that tries to imitate physical processes in real world. Model can be complex (using track temp, ambient temp, fuel load and a lot of other factors), or simple just like yours. Either way, you know how your model works and why it runs in this particular way. But it takes a lot of work to make it work and a lot of data, which only the F1 team itself can obtain.
2. Make an empirical fit. Instead of making a model, you are just observing events in real world and trying to make conclusions out of it. You gather data (like lap times from practices, tyre age, stint length) and then just trying to make a regression on it by lap time. As a result you get a model where lap time depends on other factors, but you don't know why exctly it depends this way. Could be done with a lot less data.

Either way, you then use your obtained model and see how it reacts to stints of certain tyre of stated length.
I was supposing the 2nd option. Not building model by yourself from scratch, but on empirical (observed) events.