NS FIT works well when it is treated as a plan.
It works badly when it is treated as the thing you suddenly start after pretending the window still has loads of time.
The good news is that the structure is simpler than people think.
Most NSMen do not get confused by the IPPT or NS FIT window because the rule is advanced. They get confused because the wording sounds more abstract than it really is.
So here is the plain-English version: your annual window is basically your birthday-to-birthday year. Once that clicks, the rest becomes much easier to plan.
The real value is not just understanding the rule. It is realising how that rule affects when you should test, when NS FIT becomes useful, and why so many people end up feeling "suddenly rushed."