When you do your test or practice laps you will see a % wear. If you practiced with different compound you should be able to see the difference. For an example, SS could be at 16%, S at 8%, M at 4%, H at 2%. That is how much the tires wire in their first lap. It's up to you to decide if you want to take the tires down to 50%, 40%, 33%, 30%, 25%, 20%, 10% or 0%. I'll give you 2 tips; you DO NOT want to go to below 10%, and tire wear is non-linear while first laps are at 8%, the next set of laps will be closer to 7% and on and on.
You only get 5 laps per car per race to practice. It's best to try and practice either right before your race or study the projected real world weather at the real world track and do your laps when the weather is a close match (this is easier when you are not racing every day).
You cannot tell the driver what lap to stop on, but if you go back to your practice laps you will see fuel consumption. When you decide how many laps your tires can handle, you just multiply that number by your fuel consumption and you know when your driver will stop. Note that there are 5 push levels and fuel consumption during practice is based on the middle option or "push 3".
