I do the sugar by weight to the liquid, but it probably works out about the same as yours EJ. I use between 76 and 80% by weight, so if the liquid weighs 1000g then the sugar used would be 760 to 800g. I think your way works out to 80% so maybe you could cut it down a little but don't think you should cut down much more unless you have a refractometer, and can measure the sugar content in the fruit juice accurately.
I don't use Certo or any other commercial pectin additive, don't see the point as if the pectin is present, and released, the jelly will set anyway. And it's SO expensive !!! I've read elsewhere on here that things have a "boiled" flavour unless you use pectin, afraid I've got to disagree with that one, none of my jams and jellies taste "boiled" and nor will anyones if the fruit is cooked correctly first and the jelly or jam isn't overcooked afterwards.