how you are thinking... the more of the super soil, like the 100%, makes it last longer, is not quite right. the roots use what they want, and your soil has all the nutes they want. so as long as the roots have room, they "eat" what the soil contains. no matter if it has lots or just enough, they eat exactly what they need. they make lots of roots and then you will need to up-pot again. not because it "stopped working" or you "used it up" and just need to add stuff. it is because there is not much soil left, the roots break it down, and there are lots of roots, so the water just rinses through it - your plants will look like they need nutes, which they do, but through up potting, not through dumping stuff in them. you are trying to combine two methods, and that is fine, but i would figure one or the other out before blending them.
the 1/3 is because that is what they need, it is not exactly a more is better scenario. if they use up the 1/3 they will have filled the pot with roots. any more and the same results, they fill the pot up with roots - 100% or just what they need have the same result because the plant will only take what it needs when it needs it. when that happens, you cant just add stuff to the top, or dump stuff in their water, the roots cant drink, the water doesnt have anywhere to hang out for the roots to take sips. so putting 100% or 1/3 doesnt change what the plants take, they take what they want
if you use another method, not super soil, and you do the "add nutes to water then feed the medium" method, that is a different thing. (or any other way of doing it) you control what is available to the plant and for how long, if the plant needs more it tells you and you give it. with the "super soil" it has it all, the plant takes what it needs. so if it tells you something, it is most likely different than what it tells the guy feeding stuff. mostly what it tells you is "up pot me", or "omg, there are bugs!" it wont tell you it needs cal or mag or anything, it has it, as long as it has the soil, it has the good stuff in the soil. the problems dont come till it uses up what soil is there. it doesnt just use up the nutes and leave you with blank soil, it breaks down the soil as it goes.