It sounds like a good theory, but, I don't see any consistent way to make it actually work.
I can pop a 1/72 scale jet aircraft together, with all the parts count you can ask for,,,and get it to the paint stage in a week or two. I can also feel pretty comfortable in my ability to "get it together right". That would make me a "4" in the 1 to 4 range of assembling kits.
But, cut the parts count in half, and make that a model of a 1/35 Joseph Stalin tank,,,,,and I would take far longer and be less secure in my finished result, that would make me something like a "2 as a tank builder, building a level 3 kit"
I don't see any way to be able to equate a level 2 across the board for tanks, planes, cars and ships,,,,,,,and that is what you would need in order for Grandma to ask Little Johnny what skill level he is when she goes to buy him a birthday present. Because, let's be honest,,,,,,we all ignore those boxes on the sides of the model when we buy for ourselves, if we want a new tool Tiger Moth, and the box has one inside it,,,,,,we will buy it,,,because, deep down, we all "know" that we are better than those numbers at saying how well we build.
I had the same trouble with the old kit boxes that say "age for 8 years" or "age for 14 years" on them,,,,some of them I have only had for 6 or 7 years, which means they aren't aged enough just yet.
Rex