peak (or nadir) would be the lowest number you catch in a cycle AMPS or PMPS, however it can vary, some kitties get it early, like around +3 or +4, and some get it later in a cycle, like around +9 or even +11.
It helps to do a curve once in a while on a day you will be home. That will also help you catch her peak/nadir.
yes, testing before shot is advised, most especially when you start to see lower numbers thru a cycle.
when you come up to your AMPS or PMPS, test, shoot, feed is what is encouraged, as close to the +12 hour as you can...sometimes it takes about 15 minutes to get it all done. As you get more and more use to it, and comfortable with the pre-shot numbers, you can bring that time down to 5 minutes.
are you keeping daily track of your readings? Like in a notebook for example?
you're always welcomed to post your numbers daily, that will help get you guidance, ok?