Yes, that is my pot. I was more concerned with which metal was primarily skimmed off in oxide form. But probably tin then? The bars available seem to be either SAC305 or pure tin. So if anything else than SAC305 pure tin would be the candidate.
In total I've added the full 2.5lb bar by now (roughly 1kg).
Say that my pads are 1mm diameter (including the header pin filling the hole), 40 per board, and I've done 1000 boards, then that equates to about 125000mm².
How thick is the plating? I have no idea, say 1µm. If we assume all the plating dissolves we're at 125mm³. In pure gold that is 2.5g. Dissolved into 1kg that is 0.25%
Some of those contaminants would have left the pot in joints, which also the base alloy have done. I'm not going to try to make up a differential equation based on shaky assumptions over that =D
I have no idea how off that calculation was, what concentration of gold is bad, how much other junk the chinesium headers release..
I think your advise to simply add SAC305 until the solder stops behaving is sound. Nothing I build is particularly critical.