That's really the ideal way to measure. Maybe not the most scientific seeming, but it's scientific enough. People and plants have similar heat tolerance, so if it's too hot for you it's too hot for them.

Warm is okay, but if it's the least bit hot you want to raise the lights.

This is my first post and the first time trying to start annuals by seeds. What happens to the plants if I dont keep them so close to the top of the plant?

They develop long stems and the distance between leaf nodes increases significantly.

Mike

And then they fall over, the stems thin to the point that they can't carry water any more, and the plants die.

Give them light. Nobody wants leggy seedlings. :)

Plants that don't get enough light "stretch" in an effort to try to outgrow their neighbors. In nature the sun is a pretty consistent light source so if a plant isn't getting much light it can only assume that this is because a taller neighbor is casting a big shadow.

So the recourse is to grow faster to get above this plant and find some tasty fresh sunlight.

Basically the plant grows longer and longer stem segements to maximize vertical growth at the expense of stem strength, thickness, and leaf density - all factors that improve yield, health, and vitality.

Short, fat plants are what you want to see.

Now exactly how much light you need to accomplish that depends on the plant. If they prefer direct sunlight you need a LOT of light. HIDs. If they're plants that like shade you can get away with fluorescents. You've got HID's so as long as you keep them close enough (using the aforementioned back of the hand method) you should have no trouble with stretching.