You would notice two separate measurements under this on the grading report of your large diamond engagement ring; the first one gives the actual depth in millimeters, while the other is a percentage value. The depth percentage is a heavy influencer in the value of a stone.
The farther the depth proportions stray from the optimal, the more dramatically it affects the shine of the stone. Generally, if you have something between 57% and 66%, that is seen as poor for a round brilliant cut.
This varies with each cut; for instance, a princess cut stone can have a depth percentage between 75% and 77%, and it would be very attractive. The same on a round brilliant diamond would yield far inferior results.
On the industry-wide level, there is no commonly agreed-upon ideal depth for the round brilliant cut. While some say the 58% to 60% range can deliver the best possible, others think the 62.9% to 59.5% range is a far better bet. There are vendors that try to mislead customers in this regard, using a much wider bracket and effectively hedging their own bets.