When you fill up your car
You will hear an audible filling sound when you reach almost peak fuel capacity
It's kind of hard to explain, but its just like filling up a jug and the sound it make before it overflows
Listen for that so you don't gush fuel everywhere

Also kind of calculate the kms driven
So you've done 420kms - you'll need roughly 35-39litres.
So you won't overfill - hope this crappy explanation helps