The bottom line is that there is no perfect nutrient level for ‘corals’.
Think of them like plants...countless different kinds that have countless different requirements for survival. Within those requirements are ranges. The greatest success is when you can find those things that all touch somewhere within their respective ranges...imagine a Venn diagram.
Each of those ranges has an upper and a lower threshold, with ‘optimal’ being somewhere in the middle. OK, so from an NO3/PO4 perspective, maybe optimal for a number of SPS is below the lower threshold of Euphyllia sp. corals.
Does that mean we can’t keep them together? Of course not. But for both to thrive, we have to find that sweet spot. Somewhere in all those ranges, there is a spot that will let most corals survive and do well. Finding it is the trick to reef keeping. But finding it involves looking at your coral and how it is responding to the environment far more than it does hitting a target number.
Now, in regards to the above as it pertains to nutrients, I’ll say this much: I would rather be too high than too low. Too low causes more problems, IMO.