You might have misunderstood, and most likely I did not explain it well enough.
When I'm boiling mine down, I'm boiling mine down. For the most part, I'm simmering mine on top of the woodstove, but when I'm finishing it off, or if I'm getting behind, I put it on the gas stove and put the fire right to it. Rapid boil. The formation of niter is a necessary evil (or by- product, really). The temps get up into the 220+ range, ALWAYS.
The filtering for niter comes after the final boil. I settle mine, then decant, then warm and filter the "niter rich" material left over. Some processors filter right off of the final boil. (I still formed niter after trying this, so I stopped. Probably my poor filtering was the problem.)
Once the syrup is clear and ready for canning or bottling, NOW is when the time is critical not to exceed the 190 degree mark. If you do, you will form niter again and be back to cloudy syrup.
End of last season, I was behind by about 80 gallons. Not having a big evaporator, I put 2 big hotel pans on the gas stove outside, hooked up to a 100 lber, and then had my
turkey fryer on a 20 lber, and I went after it HARD. Took my 2+ gallons of finished syrup and settled in glass pickle jars. Decanted it and canned the clear (after re-warming to 185). Then I took the "niter" syrup, warmed it and filtered it as best as I could to clear that up. Settled, decanted, filtered, settled, decanted........ until tired of making syrup and tossed about a cup or so that was still not quite clear enough.
Hope this explains it a bit better.