I'm hoping people who keep track of such things better than I will respond. My vague impression is that they drink more than you might expect, Especialy once the seedlings start to grow.
One of the issues I have with bottom watering is that you can't
tell if they need water by the way the soil feels at the top. If you force the soil to saturate to the top every time you water from the bottom, sensitive plants will die from overwatering, and dampening off is more likely. You only want to add enough water to the tray that the containers readily soak up. So with an automated system, it's impossible to see exactly how much they are taking up.
But if your seedlings are alive and well, then basically, that's how much water you need to add. -- CONGRATULATIONS, YOU FIGURED IT OUT
-- Other factors are temp, humidity and evaporation (fan, etc.) if there is no leaks. Also make sure nothing else that could be soaking off the moisture is touching the wicking pad.