It;s 510.5/1.1 = 464ml
Since the ideal tank has exactly 1000ml, you can divide 464 by 10 to get the percentage. (46.4% full)
To make things easier you can multiply 10 with 1.1 and end up with 11.
Of course everything is an approximation. It’s not “exactly” 1.1g/ml and a full tank is not “exactly” 1000ml.