RQS Apollo F1

I have massive problems with overfeeding nutrients. Last week I assumed the yellow leaftips were leafburn, but it turned out it was nitrogen toxity. Although I fed only 50% of the recomandation of the manufacturer, measuring the rinsed through water, I measured an EC-value of 3.3. So I rinsed the plants until the EC level of the rinsed water was 1.6. I wonder if the brown spots on the leaves is also a sign of overfeeding, or a magnesium deficiency.