a question on timing

[starte]

Hey Ron, great site!

I built the 11 plant garden and it's working fine but I have a question on timing because I've seen a dozen different answers. I'm currently using grow rocks supporting a 1" rockwool starter cube with my "tomato" plants and my timer is set for 15 on and 1:45 off because I want the rockwool to drain well. Do you think 15 on 45 off would be too much?

I've also noticed a pinkish brown leaf tip (just the tip) here and there, on my seedlings, but other than that the plants seems ok. I don't have a TDS meter yet so I'm just winging it. Is that from over fertilizing the seedlings?

Thank you for your time.