I ignore the trough thing altogether on the 1588 because I can't see the troughs for the automatic egg turner anway. Whenever I add water, I just snake some aquarium tubing down through a top vent and through an empty egg holder. Then I use a 60cc (ml) syringe to squirt water through the tubing down to the bottom of the incubator. I usually add 60-120ml of water at a time. I used to warm the water in the microwave before adding to the incubator, but I found that the 1588 temps don't fluctuate much when a smaller amount of water is added. I like to add smaller amounts of water more frequently as it seems to keep the humidity more constant. Under my conditions I add 60-120ml albout every day to every other day, but I'm checking humidity at least twice a day.
I check my humidity with an inexpensive electronic humidity/thermometer gauge. I watch and add water each time I notice that it drops a few percentage points below 50% (first 18 days). I try to keep it between 40-50%. I've noticed that the amount of time between adding the same amount of water varies with how many eggs are in the incubator at the time. The more eggs in the 1588, the more stable the humidity. I assume that's because with more eggs, there is more moisture to slowly diffuse from the eggs. I move my eggs to another incubator for hatching (Little Giant) in which I keep a plastic, plant catch saucer filled with water, decorative stones and a sponge to keep the humidity above 65% for hatching.
I've had fairly good luck with my hatches in the 1588 as long as the eggs are fertile.