Ebb and flow watering frequency

Hi all,

I've just built a basic ebb and flow system. I have a timer plug that works in 15 minute increments, my grow tray floods to the desired level in 10 mins and is completely drained in roughly 12 mins.

I'm just busy germinating trying to plan things out and I'm looking for some advice on how often I should be flooding the tray? I was thinking one 15 min increment every 2 hours (baring in mind some parts of the root will be in water for roughly 22 mins). I'm growing in Hydroton with a small piece of rockwool to support the intial growth from seed. Can anyone advise?

Also wondering how long after I have transplanted the germinated seed into the pots I will need to start adding nutrient to the water? I was considering 2 weeks of just water initially and then bringing the nutrients in slowly?

Only other thing which I need clarification on is what is the general rule of thumb regarding distance that a 400W HPS light would need to be from the top of a plant in an open area (indoors) to ensure minimum burning? I don't have a lot of height to work with but someone told me you can get your plants to grow outwards more and remain fairly low.?

I flood my girls 3x per day at 15 minutes, or every 8 hours.

Stick to your plan on introducting nutes slowly. I like to give them plain pH'd water for the first 3 weeks from seed.

Keep the light as close as you can without burning the leaves. Start at 12" and place your hand near your plants to check for heat. If it's too hot for your hand, you can bet your plants will suffer. Use a good thermometer too. Try and keep temps below 80. Have a small fan blowing between your lamp and plants. This will stimulate the leaves and thicken stems while keeping temps cool at the same time.

For light placement, use that thermometer to take temp reading at different distances from the light to make sure u don't just sick em in there and fry em. If u got a good aircooled hood u shoud'nt have any probs, but its best to be safe and not fry ur plants.