The plate test is what I use with Liebowitz's instructions . . .
I made sweet cherry jam with his instructions. I had no problems with jelling BUT you DO need to use a wide, open pan for the quickest evaporation. Also, a larger batch of jam will need an increase in pan size for best results. A tall, more narrow pan will mean a longer boil time, which could mean a more cooked, caramelized jam. Big, wide, open pans will give the fastest, most fresh tasting results.
And I agree, jelling in a mound on a spoon WOULD be too tight. I have made added pectin jams, especially blueberry, that have set up *solid*, almost like a jelly candy. I now add at least 1/2 cup extra blueberries to added pectin blueberry jam. Of course, we tend towards more dry blueberries out here, at least when they are fresh.