Hey,

My process is to draw it off the evaporator as close as I can to finished syrup, the bubbles will to look very glassy, small and tight ( almost like a foam ). If I get it right, I can open my valve at the back of the finishing pan and have it trickle out constantly.
I have a 5 GAL / 20L De Laval stainless milk container that I use to hold the syrup until I have a reasonable amount.
Then I will take the still very hot syrup and pour it through the heavy cotton filter with a couple of orlon pre - filters ( I do this using a stainless canner that has metal bands to hold the filters in place. ) from the canner I will fill up my Hydrometer cup and take a reading ( adjusted for temperature).
I usually aim for 66.5 - 67.0 Brix, I find the syrup has a pleasant thicker feeling on your tongue higher in the range.

I like to filter it first before the first reading, because I feel like the presence of niter / any other contaminant might skew the Hydrometer reading a bit. Also, double filtering will ensure crystal clear syrup.

Then I will put it on the stove and bring it back up to 190 ish ( if at perfect density) , pour it through another set of clean filters, back into the canner and hot pack immediately to ensure sterilized containers.

If not, boil more if on the stove up to the appropriate adjusted temp (boil water and see its boiling point on the day and add 3.94 C or 7.1 f), once reaching a bit more then that temp because I like my syrup thicker, ladle some out, hot test for density, filter and pack.

As long as the last thing you do before hot packing is filter, you are pretty much guaranteed to have niter free syrup and if you have pre filtered the syrup before bringing it back up to temp, and your filters are clean and you haven't had to boil it too much to bring up the density,
it will race through the second filtering and packing at 190 f is no issue.

Good Luck,

Jason