Never been on a tanker but I'll have a crack.
could it be as simple as
final loading rate (i.e. Xm3/h) divided by 60 to get m3 per minute.
Multiply by 10. This is the tank volume left for when you are 10 minutes from completion.
Substract from current cargo volume left to load. Divide this by current loading rate. This is the time left until you reach the point of 10 minutes to completion.
Seems like common sense basic maths. Again never been on a tanker.