Please help me with plant problem.

My friend is growing just one plant. Its under one 150 watt hps light fixture in a growbox. The light is approximately 15 inches from the plant. The temperature stays around 84-85 degrees. Soil is Sta-Green which contains:nitrogen,phosphate,potash,,potassium,canadian peat moss, and perlite. Has proper ventilation and a fan flowing fresh air over and towards the plant. He waters every 2-3 days when the soil is real crusty, how much water should he pur into it?

The plant is exactly 20 days old, its approximately 5 inches tall with 25 visible leaves, Is this good or bad?

The problem is , maybe 3 of the leaves has very small yellowish spots on the outter tips of the leaves, and the other leaves look a pretty green healthy color. What could be the cause of this?

Yellowing/browning leaf tips is usually nute burn. It's happening to leaves on the bottom? You should probably give them a light flush with pure water. Five inches tall and 25 leaves sounds awesome.

When mixing soil, the ideal amount of water is achieved when a handful of soil drips a few drops when gently squeezed and that's usually what you're aiming for when you water. More water will mean less air getting to the roots but a good soil mix should only hold so much and allow excess water to run out the bottom. How much liquid it takes to sufficiently saturate the substrate depends on the soil mix. If the soil gets very dry, the water can just run straight down and out the bottom. You can lose some expensive ferts this way if you add them too quickly. In that case, just water a little at first and let it sit and soak into the soil. Once the soil is a little moist, it will more easily absorb more liquid.

thanks alot for your information