Damson Plum Jam

I made Damson Plum Jam last night from some plums I bought at the farmer's market last weekend.  According to the Pilsbury Family Cookbook (1979) you can only keep plums in the refrigerator for 3 to 5o days, so I figured I should do something with them.  I didn't have quite 5 cups as the recipe called for in the Ball Blue Book Guide to Preserving (2009), so I chopped up a jonagold apple, which made a heaping 1 cup.  Here's the recipe as I made it:
4 cups chopped Damson plums
1 cup chopped apple
3/4 cup water
3 cups sugar
Combine all in a large saucepan, and slowly bring to a boil.  Boil until setting stage is reached.  (For me, this was when it started splattering against the walls, since my lovely thermometer us useless).  Remove from heat, and pour into sterilized canning jars, leaving 1/4-inch head space at top.  Screw on two-piece lids.  Process in hot water bath for 15 minutes.  Remove from water and let stand.