Okay, so I've worked this out in my head and it makes sense logically. Now I just have to express it clearly(the hard part), so please bear with me.
Say we have an unchangeable armor set, X. It is not customizable at all. We assign this a uniqueness value of 1.0.
Then we take a full set of piecemeal steel armor comprised of 10 pieces(it's just easier to work with decimals) designated Y. This also has a uniqueness value of 1.0.
If we swap out one piece from Y, we get armor Z. However Z is 90% identical to Y. So in terms of uniqueness Z=(Y-0.9). Therefore armor Z's uniqueness value is 0.1.
So in this scenario, while you have twice as many armor sets with piecemeal, you only get a 10% increase in uniqueness.
That's an interesting way of doing it. The way that Gpstr and I did it earlier in the thread was to just treat each different total combination of equipped armour as equal. For example, take a complete set of steel armour. In Oblivion, this is Cuirass, Greaves, Boots, Gauntlets, Helmet. That's one combination. Now swap out the Steel Greaves for Leather Greaves. That's another combination. So we've got 2 combinations. We treat each combination equally, even though the second one only involve a minor change from the first (just changing the greaves).
What you are suggesting is some sort of
weighting on how similar some combinations are to others. The problem is that it is not entirely clear how to do this in a completely above board way. What is the correct weighting? You've suggested that the similarity between two combinations is 1 - X/10, where X is the number of different pieces, and 10 is the total number of equipable armour slots. But you might think that some armour slots deserve greater weight. For instance, perhaps a cuirass should get more weight than gauntlets.
Another worry about how to do the weighting is not how to weight individual pieces against each other, but rather with the idea that it is only armour combinations with no pieces in common that are complete dissimilar. In one respect that is obviously true, but it's worth keeping in mind that the importance of similarity in this discussion is as something of a placeholder for customisation potential. The thrust of your proposal, I take it, is that we add up all the uniqueness, or the dissimilarity, and this tells us something about the total amount of customisation potential offered by Skyrim. But you might think that armour combinations which have some pieces in common are just as unique (in the relevant sense of customisation-making) as armour combinations which no pieces in common. This is a consideration in favour of treating all combinations as equal (as Gpstr and I did).