Mum makes marmalade from a recipe she found in a really old cookbook. Unfortunately, neither she nor I can remember *which* cookbook, as we have about 1000 cookbooks between us.

Yes, I know...

Anyways, it negates the need for vast quantities of water compared to fruit, is easy to remember as the only proportion you have to remember is fruit to sugar - and it always works a treat.
1) You start with any quantity you desire of citrus fruit. Slice as finely or coarsely as you fancy. Reserve pips and tie in a muslin.
2) Place fruit & pips in large pan (which has been weighed previously). Add enough water just to cover. Cook fruit on a gentle simmer until tender and most of the water has evaporated / been absorbed etc. Remove muslin with pips.
3) Weigh pan with cooked fruit. Subtract weight of pan from total. Add the same quantity of sugar to the pan as you have fruit.
4) Heat gently until sugar has dissolved, stirring occasionally, upon which you turn up the heat and boil till setting point is reached - same as you would with any other jam. I use the wrinkle test to check for setting point.
5) Pour into clean jars & seal while still hot.