Well for me once I mix the solution I just apply as much as I would apply water for a normal watering, which is typically just enough so that a little bit runs out the bottom of the pot and into the tray. Of course when your soil is dry, and you apply a good amount of water to it at once, it can run through the soil and out the pot without thoroughly soaking the soil. Give it a few minutes though and it will be soaked back up into the soil as the moisture spreads evenly throughout, as long as the soil can still hold water. So when you have runoff that doesn't get soaked back up by the plant, you have thoroughly watered the soil. I give it a little bit of time after that to make sure I've achieved proper moisture and then I take the tray out from under the pot and empty the excess water/nute solution from it so your plants aren't sitting in a puddle and the soil can begin to dry out again, and its important for plants to experience those changes in moisture rather than sitting in soggy soil their whole lives. By doing this you can avoid overwatering symptoms as well as rotting and diseases.

Hopefully that will answer your question well enough. And just to be clear that is just my method and it works well for me. Different strains react differently to everything, and other people may have different opinions as well, but for me this method is tried and true. Good luck with everything and of course never be afraid to ask for advice. Much better to ask than end up with a dead/sad plant.