I meant it varies between who you're trying to Ken Combo.
Out of the fair, you have a lot of options, and spacing basically determines what you should go for. In the case of the Ken Combo, you just have to know whereabouts they should be so that you can hit them with a sweetspot dair. Of course, they won't always fall into the exact right spot, so there are minor adjustments you can make.
For instance, if you pull back away from them and use your dair, you'll be hitting with a higher part of the arc, so you can sweetspot the dair even if they're close to your level. You could also wait and fall for a while until they were in the right place for the dair to land. There's just a lot of very small things that you can really only get a feel for through experience.
The Ken Combo really is just a leap of faith. It seems like it'd be difficult with the sweetspot requirement, but it really isn't. It's much like Fox's waveshine combos - you just have to be confident in your own ability and know that it'll work. If you hesitate or whatever, you'll miss your chance. As soon as you hit with the fair, you should already be going for the doublejump and spacing yourself out for the dair.
Of course, if they're too high and they don't fall fast enough for you to wait until they get to the sweetspot range, you could always just reverse Dolphin Slash them for the kill instead.