I think the "Change tires" setting should be only possible to set to "yes" or "no", because in real world, your pit crew can't tell exactly, in percent, how much your tires are worn.
You will actually find out that if you always do planned pitstops, there isn't any point of this. I'll try to explain. If you pit with 50 % ware, and 60 % setting, and your tires don't get replaced, and load fuel for 50 laps, you must do another pit stop before you run out of fuel, and that will mean taking even more fuel on board, which will mean more tire wear and less speed, etc. So it will be faster to always replace your tires, and just calculate your fuel well.
You will actually find out that if you always do planned pitstops, there isn't any point of this. I'll try to explain. If you pit with 50 % ware, and 60 % setting, and your tires don't get replaced, and load fuel for 50 laps, you must do another pit stop before you run out of fuel, and that will mean taking even more fuel on board, which will mean more tire wear and less speed, etc. So it will be faster to always replace your tires, and just calculate your fuel well.