Constance, you're right in that that's the way it used to be done. But I saw some studies of the % of botulism in the early 1900's as compared to -I think it was 1995 - and the incidence was way higher. So even if you and your family didn't have any bad experiences, the risks were greater.
And I think the precise measurements and ratios of ingredients in present day preserving are a result of many years of gathering information and 'experiementation'.
Remember, in our grandparents' day, their cooking knowledge was passed on from generation to generation - even tho it may not have been '1 cup of this, 1/3 cup of that', they knew from watching and helping how much vinegar/sugar/acid/ to add. Unfortunately, in our 'modern' world, we don't have that empiric knowledge passed visually and verbally down to us, so we need to rely on tested recipes.
The other thing is that 'food science' has developed so much more; and we're also not using as many ingredients grown or raised by us as our elders were, hence the chance for contamination is much higher.
Geebs - You can use other recipes than the ones Ball has in their book - Just be sure to follow the ratios they have; I speak from knowledge, lol I had a recipe from a newspaper, I think, for strawberry/ginger jam, followed that recipe to a T, and the jam didn't set. When I compared the amount of ingredients to the Ball Book's recipe for strawberry jam, I saw that my recipe didn't have nearly the sugar to fruit ratio that Ball's did; since sugar is needed also to 'set' jam, that's the reason mine didn't work.
Just my early morning, 2nd cup of coffee observations!