[Delphinus]

If it were me, .. the only thing I'd do different is start with a target of 5ppm not 10-20. But that's me, I guess it's where your comfort level is. Think of it this way though: if you have 0 of anything (NO3 in this case, but it applies to any measurable parameter), it means that consumption of said parameter matches production, ie., not so much that you don't have it in the first place.

So a consistent level of any nonzero value, maintained by dosing, means you're adding it, and it's available in some form of surplus to the consumers. Ie., if a clam is consuming nitrate, and you're adding nitrate and testing nitrate to ensure it doesn't go over 5, then that clam has nitrate available to it just as much as it would were the level 10ppm. But at 5ppm, things that may be stressed by NO3 will be less stressed than with 10ppm or 20ppm.

I don't know if I'm making any sense, but that's how I would look at it. There may be a benefit to going higher, I don't really know, but my intuition tells me that 5 should be just as good as 10 and would be less obtrusive to your other inhabitants.

I had some bleach spots in my larger squamosa too for a while. It took nearly two years for them to blend in and disappear - very long time. I've never had zero nitrates in my tanks, even right now with my cube tank I have to dose Nitrate Destroyer to maintain <2.0ppm and I have 12 clams. So I don't think nonzero nitrates helped it recover in my case. However my tank is a larger water volume and with fewer clams than your tank (I topped out at 14, but am now down to 12 due to some recent losses), so maybe that accounts for the difference.