Untested - Jelly testing

1 - I have had no luck with making preserves using liquid pectin. I don't know why. As far as I can tell I follow the recipes to a T and then it just doesn't solidify. I'm saving here another jelly recipe I've had success with, to see if I can transpose the techniques somehow. The difference is that I used fruit with naturally occurring pectin, while this other stuff is just herb jellies, and have to have pectin added. If I can make this work for herb jellies, then I can standardize the technique for anything!
What I'm most interested in, I think, is the jel test in point #3.
2 - Years later, I tried again, and I think I found the solution. See point #4 (newly added).

2.2 lbs. Mirabelle plums, cut in half (do not remove the pits and skins)
3 cups of sugar
the juice of 1 lemon
4 sprigs of rosemary
1 tsp. vanilla

1. Macerate the fruit with the sugar and lemon juice overnight in a covered ceramic bowl.
2. The next day, dump the macerated fruit into a soup pot and add the rosemary and vanilla extract. Cook, covered, over gentle heat, for 30-45 minutes.
3. To test that the jam is ready, keep a plate in the freezer and drip some of the cooked juices onto the plate. If the drips gel to the plate (don't run when you tip the plate sideways, the jam is ready. Otherwise, keep it simmering.
4. When the jam or jelly starts to boil, clip on a candy thermometer, reduce it to a rapid simmer and continue to cook, uncovered, stirring occasionally, until the thermometer reads 220F/105C. This can take upwards of 30 minutes, less if you add more sugar, longer if you have less sugar.
5. Once the jam is ready, strain the fruit through a sieve with a wooden spoon until all that's left are the skins and pits and rosemary stems.
6. Heat some jars in boiling water.
7. Heat the jam on the stove until just starting to simmer, then pour into the hot pots.
8. Screw the lids on tight and leave the filled pots on the counter, well spaced apart, to slowly cool. You'll hear some lovely 'pop's as the seals engage.