Wow, you are going to put my pre-CU knowledge to the test
The truth is I know a lot more about NGE crafting than pre-CU. I played NGE far longer. I was still learning all of the game in pre-CU. By the NGE I had a better grasp on all aspects of SWG and could zero in on crafting. This site is essentially an NGE crafting aid. This version did not even start till after the NGE (a long story). After SOE shut down, the only SWG left was the Emu, and it is pre-CU. So we had to reinvent ourselves as a pre-CU site catering to the Emu servers.
This is what I think I know about pre-CU (I might need to go to the Emu devs to verify some of this): Pre-CU does not adjust for resource caps (the CU and NGE did). In pre-CU the resource stat is used unmodified. So in your example, that JTL copper with 904 SR will indeed be 904 in the calculations and not reduced to 800 (the cap for copper). In the CU and NGE it would be different. The 904 would be first reduced to 800 then adjusted to 1000 and calculated from there. But not in pre-CU. Again, there is no adjustment for resource caps in pre-CU. Stats are used as is.
Because of this, that JTL copper with will be better to use than a Polysteel Copper of 800, by quite a bit. In the CU and NGE they would be identical.
So why do we keep "parent caps" if they don't mean anything in pre-CU? Or why list % of cap when it doesn't mean anything? For two reasons. One because most of us here at SWGCraft always believed that SOE screwed up letting JTL resources go over their "parent caps" and we hoped that one day SOE would come to their senses and correct the mistake. But the biggest reason is that people playing a CU or NGE version of SWG will need to know, because it will matter to them.
Edit: Thinking about this a little more, in the CU and NGE it might not be a case of the 904 SR being reduced to 800. What probably happened is the SR cap of 800 was adjusted for (stat*1000/cap) and 904 produced a result over 1000 which was then reduced to 1000 for further calculations. It would make more sense to do it that way.
Doesn't change what happens in pre-CU, either way.