Jason. Perhaps you are lucky and your frogs are pretty hardy, however low humidity is very dangerous, especially for babies. Yes older frogs can tolerate lower humidity to some extent, and if conditioned properly can be aestivating, but babies- new environment- big tank- low humidity situation might be very dangerous.
as for dart frog analogy, well may be you should read about them a little, they require way higher humidity then pacs and will suffer in 75.
Pacs are not as hardy as you think, they are susceptible to lots of problems, especially in stressful situations and the younger they are the more sensitive they are. Proper care is required, proper gauges are must for newbies. With experience you can cut corners and go by frog behaviour, because you can detect any small changes in frog wellbeing and react accordingly, new to the hobby people can't have that luxury.
Covering the top - it's trial, and if you live in higher humidity climate it is different from what people in very dry climates need to do to keep any humidity in their tanks. 2/3 top coverage won't work for me living in ambient humidity of 10% I'm going for almost full top coverage or taller tank. In general the taller the tank the easier it is to maintain humidity on a lower levels.