if you live 71 mi from a river does it make sense to say you live about 80 mi from the river
No.
No, it doesn't make sense to say that you live about 80 miles from the river if you actually live 71 miles away. Saying you live about 80 miles from the river would be an incorrect statement.
When measuring distances, it is important to be precise to convey accurate information. In this case, stating that you live 71 miles from the river provides the correct information.