If you want to use less sugar in a jam recipe, your best bet is to find one already written to be low-sugar. Jams are finicky and altering the ratio can result in a jam that doesn’t, well, jam. Most experts recommend using a 1:1 ratio of sugar to fruit. So, if you’re adding a pound of fruit, you will need about a pound of sugar. My strawberry apple jam recipe goes a little lighter on the sugar (about 1.5 lbs sugar to 2.5 lbs fruit) so, I wouldn’t attempt to go any lower. It sounds like a lot, but remember, sugar acts as both a preservative and an agent to properly help the pectin gel.