I hit a similar issue when I did the Viennese Whirls, so I decided to knuckle down, do some reading, and then do a series of batches to trial different methods. It was a good learning experience, and I think (for strawberry jam at least) I've made some progress.
So, a few facoids that I've come away with;
1) Lemon Juice is added to jam not just for taste, but also to assist with pectin setting. There is all sorts of science behind it (I didn't read it all) but the long and short of it is that pectin sets better in an acidic environment. The acidic environment also helps preservation, as it kills more things (less of a concern for the small batches you tend to make baking, but something to note if I ever do a huge volume)2) Strawberries are low in pectin, so you probably need to add some to the mix. This can be done by adding some high-pectin fruit (apple seems popular), adding pectin directly, or (most commonly) using jam sugar, which has pectin and citric acid added (I didn't know about the citric acid, but reading the ingredients in Waitrose on Friday evening, a couple of things clicked)
3) While the most common sugar:fruit ratio is 1:1, you can add more fruit than sugar. 7:10 seems popular, and I've trialled it, though yet to check on the set.
When it comes to setting points, there are all sorts of things about dropping liquid on cold plates, and "wrinkling" it...however I opt for a slightly more scientific method, and use a temperature probe, and look for a temperature of 104'c. pectin doesn't like over-boiling, so use of a temperature probe should, in theory, allow me to rapidly make small batches of jam...this was what I wanted to test.
So, while watched La Vuelta this afternoon, I did 4 small sets of jam, in a variety of volumes. I'd done 1:1 ratios of strawberry and caster sugar for the Viennese Whirls earlier in the week (too runny);
1) 100g of jam sugar and 100g of strawberry
2) 70g of caster sugar, 100g of strawberry and 5ml of lemon juice
3) 70g of jam sugar, 100g of strawberry and 5ml of lemon juice
4) 140g of jam sugar, 140g of strawberry and 5ml of lemon juice
All of them were boiled to 104'C, then decanted into a small jar and left to set. I did the traditional "poke test" once they were done to determine the set. The caster sugar one (2) was still fluid, while the others were all similar.
I used (4) to make a second batch of Viennese Whirls...as you can see above 140-150g of fruit is roughly the right volume for a single jar as well (also good to know).
There is a handy chart here that shows the different pectin and acid levels of various fruits. It's no substitute for a bit of practice, but at a pinch it can help with determining the right sugars and acids to use when making a jam.