Well as I mentioned,
"Once the sprouts have started to get their first set of true leaves, I mix the
nutrient solution to between 1/8 and 1/4 strength. To make things easy I just take a gallon water jug, make that full strength. So I have a gallon of full strength nutrient solution (sometimes I pH adjust it, but most of the time I'm too lazy), then just dilute it as I need. Store it in a dark closet to keep algae from growing.
I gradually increase the strength up to about 1/2 strength. By then the seedlings are about big enough to be placed in the
hydroponic system where I still use about 1/2 strength nutrient solution. Once placed in the hydroponic system I make sure pH levels are right. When the plants get a little bigger, I'll increase the nutrient strength to 3/4. I don't usually ever make/mix the nutrient strength higher than 3/4 of the manufactures directions for a full strength solution."
I tried to word it in a more general description, rather than specific to one type of seed/plant. As far as what I do, I still generally fallow this same general rule of thumb. But not all
seeds grow at the same speed or size, so that's why I don't mention a timeline or specific sizes for guidelines. Other than I start with a weak nutrient solution once the seeds have started growing their first set of true leaves. Faster growing plants will naturally get bigger faster, thus should have their nutrient strength increased earlier than slow growing plants. Tomato plants just happen to be a fast growing plant (under the good conditions). Witch leads me to another variable that is the conditions will greatly affect the seedlings growth (air and root temps, humidity, light etc.). Environmental conditions are more important than exactly when to increase nutrient strength, or even how much to increase it to me. But once you start a few plants and see how they respond to any changes, you'll have the feel of it quickly (even have your own preferences).