I accidentally added too much sugar to a blackberry jam one year and it was inedibly sweet, so I made a wine with it. From memory I think it was 1.5kg of jam, of which half of it was sugar, so I made it up to 1kg of sugar. It brewed up great and cleared no problem without pectin. It was a bit thin, mainly because I didn't use jam in it. As a general rule with soft fruit wines, you use 2kg of fruit with about 1kg of added sugar, but I didn't get enough fruit in there, hence it tasting a bit thin.
I'd go with:
2kg jam
Juice 2 lemons
1 mug strong black tea
Sugar (see below)
Yeast and nutrient
Boiled and cooled water to one gallon
Have a taste. If it tastes too watery then add more jam. Then aim for a total of 1100-1200g of sugar. Look at how much sugar is in the jam and adjust the total amount accordingly with white sugar. Get 3.5 litres of it brewing. Once the initial fermentation has calmed down, add the final 1.5 litres.