Are there alternative ways to test fruit preserves for doneness?
Have you tried using a candy thermometer and testing the jam when it's at 220°F?

An alternative to the method that you use is to use a spoon and do the 'two drop test'. If you dip a cold metal spoon into your jam mix and then lift it. When the mixture is only just boiling it will drip off and be light. As the mixture continues to heat the drops that fall from the spoon will be heavier  When the two drops form and fall off the spoon it should be ready. I'll be honest though I much prefer the method you use and this is just an alternative.