The total fruit at the start was approximately 4 cups and yielded 3 cups of jam. It’s best to have at least an extra 1/2 cup in the canner just in case.
There are two methods that I use to test if the jam is done. The first is to put a small plate in the freezer. When you think the jam is ready, smear a teaspoon of jam across the cold plate, then run your finger through it. It should not run into the gap when you tilt the plate.
The second method is to use a wooden spoon. When you think the jam is ready, lift some into the spoon. Let it stand for a few seconds (don’t blow on it, but you can roll it around). Then pour it back into the pot. The drips should be thick and heavy if it has gelled. If it’s fast and runny then it needs more time on the boil.