My assumption is that he’s referring to advancements in your ability to handle adversity. If you’re able to handle more and more stressful situations, whatever you’re doing is working. If not, you might need a little more, or maybe a little less.
Which leads me to answering your question:
There’s some trial and error here. But think about in like weight training progression.
If you’re training at an appropriate level, lifts should feel hard but doable. Eventually, certain weights will feel like a breeze, and you can up the weight. If you start too hard and heavy, you might be able to handle it for a while, but chances are you’ll just crush yourself right out of the gate. Similarly, going too hard too regularly will fry you.
Sometimes, you also have testing periods, usually at the end of a training block. This lets you gauge where you’re at and see how you need to adjust.
Preparing yourself for adversity is very similar. Start off with scenarios you know will stress you out, but only in a dose large enough to mildly frustrate or upset you. When you feel those emotions or states of being rising, you take a moment to pause and calm yourself down. Eventually, handling those small moments becomes easy and you train your natural instinct to be finding a state of calm. Slowly increase the level of stress you bring on yourself, working towards the same state of mind as the lesser scenarios.
However, if you go too hard too soon, you can cause your natural instinct to be one of intense panic, anger, fear, etc. You won’t know how to handle such an intense level of adversity, you won’t know how to reach a state of calm, and you’ll leave the situation in that negative state, thus training yourself to make that your base instinct.
There are moments where you can test yourself to see if you can reach a level of calm under far greater difficult circumstances than you’re normally used to, but much like training, maxing out is normally frowned upon because if you’re not ready, you can do more harm than good. In the case of steeling yourself against adversity, that harm is backsliding your progress or instilling an instinct of negativity in intense moments.