Strawberry Jam

Today I made my very first batch of strawberry Jam.

I've often made jam by combining strawberries with other fruit, especially gooseberries but have never before used just strawberries because jam makers I know say that jam with just strawberries won't set, though I'm not sure whether that's because strawberries lack pectin, or because they aren't acid enough.

Just to be on the safe side I added some lemon juice, but I still found setting hard to achieve until I added a lot of extra sugar.

My observations suggest the following recipe:

1 kg strawberries
200 ml lemon juice
1.6 kg sugar

The standard proportions for jam are equal quantities of fruit and sugar, so that's 33% extra sugar.

I wonder if extra sugar would work without the lemon juice. I may try with tiny quantities.