Place the strawberries, rhubarb, and ¼ cup water in a medium saucepan and bring them to a boil. Cover, reduce the heat to medium, and cook for about 10 minutes, or until the fruit is as broken down as desired.
While the fruit is simmering, prepare the calcium water from the Pomona's Pectin package by combining ½ teaspoon of the calcium powder with ½ cup water in a lidded jar. Cover and shake well to combine. Set it aside.
Prepare the pectin by combining 1 teaspoon of the Pomona's Pectin powder with ⅓ cup boiling water. The powder is hard to dissolve, so I blend it with an immersion blender for a few seconds and that does the trick. Set it aside.
Once your fruit is softened and ready, add just 1 teaspoon of the calcium water to the fruit (Store the leftover calcium water in the refrigerator for later), plus the pectin mixture and the sweetener. Bring the fruit back up to a boil, stirring for 1 minute while it comes to a boil and thickens. Remove it from the heat.
Taste the jam, and add additional sweetener to taste.
Pour the jam into 3-8 ounce jars, leaving about ¼" of space on top. Screw the 2-piece lids on tightly.
You can store this in the refrigerator and use it within about 3 weeks. Alternatively, you can put the filled jars in boiling water to cover, and boil for 10 minutes to can them. Check the seals and make sure the lids are sucked in.