There are three ways:
Basically they are just making it so you can cook by the clock. The only way to accomplish that is to remove all the variables (although they don't mention elevation, but they should provide a chart because it does make a difference).
The most important variable is the amount of sugar in the fruit. But if you add a ridiculous amount of sugar, it doesn't much matter what you started with. So what if it overpowers the taste of the fruit?
Well, there is another way. I put in less than half the sugar that the recipe calls for, and as soon as it starts tasting sweet, that's enough. More sugar means less fruit taste.
Jelling is a three-way reaction, among the sugars, pectin, and acid. Most fruit has some acid, and there is some in the commercial pectin as well. If the juice is not at all tart, you might add some lemon juice. But the fruit has to have some acid. For instance, if you're starting from pear juice, then you oughta just forget the whole thing and drink the juice. No cookbook has ever even mentioned pear jelly, because nobody has ever made pear jelly ... so of course I tried it. Well, guess what? I cooked and cooked and cooked, and nothing ever happened, until shazam! suddenly I had made pear rubber.
Addendum, Tuesday 1997:
I have recently been informed that somebody has made pear jelly, and also at the bottom of this page I have added a reference to an article that has a recipe for it. Actually it's pear and lemon juice jelly. Whatever -- maybe some day I'll try it again.
Back to the process. The first part of which is cooking the fruit and extracting the jelly. I'm just going to refer you to the instructions that came with the pectin, since you will probably be using commercial pectin. If you know how to get your own out of little green apples, you don't need this page.
Matter of fact, I'm going to let you follow all the instructions that came with the pectin, except for one: cooking time. If you are going light on the sugar, you will be cooking it longer. Keep stirring it and you will see it going through some changes. You test the consistency by dipping a spoon in and letting it run off the spoon sideways. At first it will just run off in a single stream, and then it will start dividing into more than one. Eventually it will "sheet" off the spoon. That's it. Turn off the fire (you are using fire, right? I refuse to discuss electric stoves) and put it in the jars. All you need to know about that is in the printed sheet too.
Let me know if you have anything that ought to be included in this page. Enjoy!
Some other nice resources I know about:
Carla Emery's Old-Fashioned Recipe Book (An Encyclopedia of Country Living) ISBN 0-553-01068-9. 70's. I thought it was out of print, but I have been informed of the existence of a newer edition, and found that it is currently available at Amazon: ISBN 0912365951
Article entitled "The Joy of Making Jelly" by Jean Anderson in Gourmet magazine, September 1985, page 62. Recipes in this article are: