The consistency of royal icing depends on many things, including the size of your eggs, but also the humidity/weather. So if you know what stiffness you want, it's perfectly valid to add sugar until it "feels right". (I usually make royal icing with meringue powder rather than fresh egg whites, which removes one variable from the equation [the size of the eggs], but I still have to adjust the sugar to get the right consistency.)
As far as troubleshooting, if you've been whipping away and it's still gloopy, by all means add more sugar. If you overdo the sugar, you'll know immediately: it'll be too stiff to mix. (In which case, depending on the quantities involved you can either just add a teaspoon of warm water, or you can whip up another egg white separately until soft peaks form, then fold it into your icing.)