Making royal icing today, for a rather overdue Christmas cake.

I always end up with too much icing sugar, based on my normal recipe which asks for 4 large egg whites and 500g icing sugar.

So today I used 3 egg whites and as much icing sugar as "felt right". Is this fair enough?

If it's taking a long time for stiff peaks to form, is this a sign of too little icing sugar? And if I overdid the icing sugar how would I tell?