1st indoor 400 watt hps grow. (help or advice appreciated)

Hello everyone, I'm new here and i like how on this forum everyone is so helpful and knowledgeable about this subject. So based on that, I would like some help on my very first indoor grow from some bagseeds I had laying around the house. So far they are planted in some sunshine mix 4, and I have 2 plants 1 week and 2 days in and they have 1 set of leaves with another set half as big, and another plant half a week in with only its first set. I have been feeding tap water whenever they feel too dry, due to my low 20%-35% and sometimes 40% humidity levels it is usually every 3 days. I have 1 400w hps hood about 30 inches above the plants, and the temperature is usually between 70-84 and 89 degrees on 90+ degree days outside. I am using a cardboard ventilation with 3 fans i can switch to intake or exhaust, I usually run intake and the room stays fairly cool. And also I do not have any reflective material up yet, it is a wooden bookshelf with only one shelf left on the bottom. I haven't been using nutrients yet just tap water which is really clean at only 63 ppm, I was wondering when i should start using my general hydroponics flora series nutes? and also if these conditions will produce decent plants or if I should do things differently. As I said in the title, any help or advice is highly appreciated and thanks in advance.

Cheers and happy growing! -1817 :smokin:

white paint will give you a good reflective surface. I use cardboard sheets so I can keep the light in close on the grow area.
I feed after three weeks, usually. Your Sunshine mix is very good but may need micro nutrients. Check with GH for a product that goes with the Flora line. Go easy on the nutrients, say 50% of the suggested amount.