"I think the difficulty with admissions is not necessarily not understanding the criteria but just not knowing, for sure, whether you will be successful or not"
But that doesn't really matter, provided you believe the system works as advertised.
I've recounted this before, but it stands repetition.
I live on the fringes of the catchment for a popular comp (comp A): we're usually the last road, or sometimes one beyond, depending on the number of siblings that year. I live in the catchment of another, slightly less popular, comp (comp B), which some parents (rightly or wrongly, in my view wrongly, don't like as much as the first one). And this being Birmingham, there's the 11+ grammars.
It's routine to hear of people not entering their children for the 11+ because "I don't know if she'll pass, and you need to put comp A first to get in''. Which is simply bollocks. There is no reason not to put down four fantasy schools you won't get into without divine intervention, one you might just and a sure thing sixth. So putting down four selective schools in descending order of entry requirement, comp A and comp B, in that order, gives you precisely the same chance of getting into comp A as had you put it first, given your child doesn't score well enough in the exam. First, fifth preference: it doesn't matter.
Endless calculations of "I shouldn't put that on the form because it's not certain I'll get in" only matter if you end up not putting down a school that you would both accept and definitely get into as your last preference. That's mad: you get allocated city-wide. But assuming there is a backstop, you don't need to worry as to how likely you are to get it.
There is no gaming of the form: you just put them down in the order of preference. It really is that simple.