Each new bean becomes a bit of an experiment with each new machine you use.
A rule of thumb is to grind just enough to fill the portafilter so the beveled edge of the tamper becomes flush with the edge of the portafilter using about 15kg of pressure.
A crude diagram:
|||||| < Tamper
| [||||||||] | < Tamper 'head' flush with portafilter edge (note, it's flush)
| .......... | < Tamped coffee
| .......... |
/ \ < Goodness pours out here
But how much coffee that is, and just how much over 15kg you need to apply in order to tamp that much coffee to get the right pressure to pull a great shot is a matter of science that you'll need to do. You might want a finer grind instead of more pressure, you have to experiment.
Start with your usual espresso grind setting, a dose appropriate for the size of the filter you're using and then work from there.
You can get grinders that auto tamp for you based on your settings, so when you hit that 'eureka!' moment, you can reproduce it somewhat reliably using the same equipment. For home use? Just be ready to experiment with the first few filters full when you get beans you haven't tried before.
There's no real 'rule' here, other than you want as much coffee as you can get in the filter with the tamper sinking flush given about 15kg-ish of pressure. It varies wildly from there.