Don't worry so much about your run off being at a particular number. Every setup is a bit different. My run off is double this and I've made mistakes that put me up to 3400 runoff. This got me very animated and upset. I flushed gown to 1600 and all is good. If I had been testing periodically I might have seen this coming.
Watch for a trend of increasing numbers, this can help you avoid problems. My experience is these plants change their demands dramatically 1-2 weeks after the 12/12 switch because they always need less water and my ppm goes up fast. It always seams to catch me sleeping like it doesn't happen every time.