I weigh my ingredients, but I keep boiling until it goes the right consistency when dripped on an ice cube, or crinkles slightly when you push a finger through it after allowing a dribble to set briefly on a plate. After that, I just dump it in pre-sterilised jars to cool down, and pop the lids on while hot. They'll seal themselves as they cool, and if you've put enough sugar in, you shouldn't need to worry about them being contaminated by air/transfer, so there's no need for a full canning-style sterilisation. That's how we do it in the UK, anyway. I've never had a jar of home made jam go off - most of my ancient ones are lost to crystallisation, not mould.