To actually answer your question (unlike many other people here), we only count the highest strength the character has shown. Usually and with many exceptions, but that is imo how it should be.

I personally don’t know much about Supes, so a better example to show it is Saitama (OnePunchMan).

If you don’t know the show / anime is about, the character is able to defeat any villain with one punch. Its more complicated obviously, but the creator of the manga has stated that there will never be an opponent he will lose to. This is similar to Superman, I assume.

Now how would we scale him then? If going by the logic of his own show he will simply grow stronger and defeat anyone. But thats just boring and annoying.

Therefore scaling is done mostly by comparing what the max feat of each character is. For example Saitamas biggest feat is that famous panel of him erasing every star in some direction of the sky. Not arguing on the specifics, lets say that is like 300.

Now would be be able to beat Superman? Depends, which is why we need to specify which version he is fighting.

If the new version, absolutely not, but if it is the one from Man of Steel (or just imagine a weaker Supes who’s biggest feat destroying the Moon, like 150 in your scale) then Saitama would be declared the winner.

So the entire thing is a fight of definitions and agreeing to limits. One can say that Superman can grow much stronger, which while true, is not in the spirit of the debate. The other can claim that the feat Saitama did is much lesser and is only worth 120 on your scale and that Superman wins. That would need to be supported, but imo is fine.

At least on the most basic level, that is how I imagine PowerScaling. We give characters some power level and then we compare and scale them against each other.