How much water for seedlings ?

as much wood as a woodchuck could chuck, if a woodchuck could chuck wood. don't drown them.. don't let it dry out. you need to change your perspective on watering. you water the entire pot... for a seedling in coco-based medium, i wouldn't go beyond the top layer changing colors as it dries out... in fact, be safe and water before that point. weight of the container is easier signal to be consistent with timing. the time between will change... plant grows bigger, drinks more and the steady evaporation rate becomes a smaller proportion of the total as you go along until a transplant, and then it just resets slightly and continues that trend. by the 2nd or 3rd irigation, i'd do my normal wet-to-dry cycle before irrigating again. since you are soilless, start fertilizing every time by the 2nd or third irrigation... if you are 1.5EC or lower (US scale), it's nearly impossible to harm them at that dose and you don't have any slowdown. once you do start fertilizing, have a religious 10-15% runoff waste water.. toss it down drain or outside... if you have a well-balanced feed, even your first run will be great. if not, you'll lean to adjust and start in a better place on 2nd grow. it won't take long with a soilless medium. we control all that it gets... no surprises once you got it mapped out. converting to ppm per molecule and total feed from guaranteed analysis is suggested... once you have a good idea of it, you don't need to calculate it constantly for minor adjustments.... but, as you learn you'll more easily be able to diagnose and eliminate possibilities as to cause of a problem... before long it'll be easy sailing and rarely look at it again. this will shorten learning curve, guaranteed. check out nutrient ppm calculator on manic botanix website. e.g. some ppl say "cal mag is required" for coco... well i provide 100ish Ca plus my hard water (so, >100 by a good bit)... when i grew in coco i never had a Ca or Mg problem... never used cal-mag or an additional calcium supplement, because it is about enough ppm's and not some magic product. i knew what i fed was enough Ca. if coco is leaching your Ca, change products immediately. that means it's not been processed properly... a good medium shouldn't steal molecules.