Actually, it's 10x to 30x... not %.
And yeah... as long as you have the flow where you need it, there's nothing that says you HAVE to have a certain amount of turnover. In fact, you don't even need to give a coral the amount of flow it "needs"... just don't expect it to thrive. Many corals (but not all) will stay alive just fine with less flow than they should have. They probably won't grow much, but they'll be alive.
The "x times your tank volume per hour" rule, like many other rules, gets outdated and not as useful as equipment changes over the years. For example, 600 gph
out of a MaxiJet is entirely different than 600 gph
out of a Korallia. Totally different beasts.