My Hovabator instructions are OK, if a little vague.  I think people's climates, home environments etc are such a wild card that you can't really start tweaking your technique until you've done a hatch or two.
Weirdly, this time I am more or less following the bator instructions to the letter and am having better success than other hatches.  Having your bator in a temp and humidity stable room is a huge factor no one ever talks about.
		
		
	 
You can't tweak, but you can give yourself an advantage, by having a set of checks and balances such as duo checked thermomters, checking either the air cells or the weight of the eggs to control humidity, knowing what you are looking for when candling.
 
The biggest problem with the instructions with these bators where humidity is concerned is that they don't take any of the variables into consideration when they throw out humidity suggestions. You can't just "fill x well with x amount of water" or throw out a number, which is generally too high.  You have to consider the climate, (10mls of water in a bator is going to give a person in a dry comment a totally different rate than one in a more humid climate.) the hatchers habits, how often do they opening the bator, do they candle a lot, are they hand turning?, the shell and egg quality and size. Thicker shells-less porous shells are going to theoretically need less humidity than thinner more porous shelled eggs, same goes with size, a smaller bantam egg will more often need a higher humidity than a bigger full size standard egg.
 
You have to basically pick a number or method, (I use a low humidity method the first 17 days) and have a way to actually monitor if that is working for you so you have plenty of time to adjust and change to get the results you need before those babies go into lockdown day 18.
 
Yes stability is a big factor and yes it is talked about though not as often as other aspects. I think it gets thrown in the "common sense" category and only pulled out when temp flunctuations or humidity changes are brought up. It is a huge factor.
 
Knowing that your temps are accurate and steady and being able to monitor humidity and know that it's workng the way it should will save a lot of "tweaking" and hopefully keep a hatcher from having to experience very poor results in the begining.  You will always be tweaking and changing looking for "better" hatches, but it sucks to start off with disasterous hatches in the first hatch.
 
I have talked to multiple people that were so dishearted at having an awful first hatch they weren't sure if they ever wanted to try again. I've even been able to guide people through that second try with advice with what I've found works especially with the styros, that have managed to have much better results and were able to find the enjoyment in hatching instead of giving up.
 
Even though I am pretty successful in my hatches overall, I still look for things to "tweak" to get that 100% hatch. (With more than a dozen eggs.) I've only done it once so far, but I am determined to repeat the process.  I switched from auto turner to hand turning then from the normal ceasation of turning on day 18 to day 14. (And I have to say, other than the two jumbo green eggs that I stupidly set with the small silkie eggs that didn't loose enough moisture and grew too big to turn, I have not had a true upside down malposition since I changed my turning practices. Coincidence??? I dunno....)