Wash your jars and rings and lay them out on a towel to dry. Place your lids in a small saucepan and put over medium heat, so that the sealing compound softens in preparation for canning.
In a 8-quart, non-reactive pot, bring the rhubarb, strawberries, sugar and water to a boil. Add the cinnamon, lemon juice/zest and salt to the pot and let it bubble gently for about ten minutes (on my stove, this means I set it to medium-high). After ten minutes have elapsed, use a potato masher to break down any large pieces of strawberries that are left (the rhubarb should have broken down nicely by now). If you like a smoother texture, use an immersion blender to puree the fruit. Add the pectin, stir to combine and let cook for a few more minutes.
At this point, dip a spoon in the jam and see how it coats the back of the spoon. You can also try the saucer test. If you get a nice, even sheet, the jam is done. You can also taste at this point, to see if you like the balance of flavors. Add a little more lemon juice if you feel it needs additional brightening.
Pour into hot jars, wipe rims to remove any spillage and apply lids/rings.
Process in a hot water bath for ten minutes.
Remove from water and let cool.