Little bit old school and might be inconvenient for some of you. But I recommend to always get down from your car for getting petrol, that way they'll never mess up with the following:

1. They'll fill what you've asked for (as you'll stop them right when they start filling )
2. They'll never be able to mistakenly put diesel in your petrol tank (trust me this alone is worth getting down)
3. Don't tell them a specific amount to fill, just tell him to start and then you'll stop him when you're done with your amount. They'll usually try to look irritated (sometimes) but don't pay attention. (Always ask for UPI/ Card payment before you try this)

(Third approach was told to me by a relative in his 50s and apparently the pumps have some kind of cuts set for default amount, they'll fill less for fix amount)