Hey there!
Checking your runoff PPM is a smart move to monitor what's going on with your plant's nutrient uptake. It's normal for the runoff PPM to be higher than the input, but a significant increase like that could indicate a buildup of salts in your growing medium.
Since your input is around 600 PPM and your runoff is at 1400 PPM, it suggests that your plant might not be fully utilizing all the nutrients you're giving it, leading to accumulation in the medium.
Increasing the volume of water you're giving until you see a decrease in runoff PPM can help flush out some of those excess salts. Aim to achieve at least 20% runoff to ensure thorough flushing.
However, considering you're in week 13 of your auto Northern Lights, you need to be cautious about overwatering or causing stress to your plant. Gradually increase the volume of water over a couple of watering sessions to avoid shocking your plant.
Additionally, if you haven't already, it might be a good idea to check the pH of your runoff as well. Fluctuations in pH can affect nutrient uptake, so ensuring it's within the proper range can help your plant better absorb nutrients.
Overall, a little adjustment in your watering routine should help rebalance things and ensure your plant gets what it needs without any excess buildup. Happy growing!