A perfectly fit ring should not come off easily when you ease it on your finger rather slide smoothly. Your precious ring should be properly sized to minimize the risk of loss and for a comfort use which won’t put too much pressure on your fingers.
Note that if you are hover between two sizes, pick the larger one. Also, consider the width of the ring you are planning to give. If it is wide, choose 0,5 larger size.
If you are planning to give a surprise gift, pick one of her/his rings that fits well and place it on the chart to measure the ring size