I've tried about every way and settled on the hydrometer... however... I would say there is significant batch to batch differences in how they come off the ladle... and how they filter. Sometimes a batch seems crazy thick, and I have trouble filtering... next batch much thinner and filters fine. All with the same hydrometer (2 actually to double check).... can't really explain it. the final product always seems the same... so maybe alot of nitre thickens up the syrup before you filter????

sometimes get a buddy to check with a refractometer and I'm usually about bang on.