This is a very good point of view by Geoffda. Knowing what you don't know is the first step in freeing your mind to learning.
You should not be messing around with your boots nor should you be self diagnosing alignment issues. I've had the training and I wouldn't even work on my own boots without supervision because I recognize that absent a significant apprenticeship (which I have not had), I am in no position to put the theory into practice. You don't even have the theory, so stop wasting your time. If you can't get out to HSS to get your boots set up properly, then leave them alone. Doing nothing is by far the lesser of two evils in this case.
There are many posts here that are giving the correct advice about footbeds feet and ankles. as one poster stated, "you can spend 1000s on boots, footbeds and alignment and still have it all wrong." A few points I think need clarification. Flat feet are never all the same. Both Ingmar Stenmark and I have flat feet. Flat feet that are hind foot, calcaneus vertical or close to vertical are excellent for skiing and are not eversion restrictive. They are not locked, you have to know how to determine this in both open and closed kinetic chain measurements procedures. This is only one aspect to finding the optimal, and achieving correction isn't always optimal, feet are not perfect engineering mechanisms. Functional evaluation has to be understood and I detect this is missing in this on going discussion and not considered in the approaches taken.
Alignment is not done inside the ski boot. In fact adding posting can cause the eversion limitation.