if you don't have a multimeter, you'll need to know the impedance of both your source and your computer's input, then if you have a cable to go from your source to the computer's input, you could fool around with a test tone and a spectrometer or anything showing loudness with an ok db value, and find out approximately where you need to set your adapter buy measuring with and without it.
redneck version if you have no idea what I just talked about: I don't know the impedance of the continental. so with the ipod(I counted 4ohm?) if you really have nothing, set the adapter to get about half the loudness(yeah not amazing precision here) of the maxed out volume level. that's supposed to be around 10db, and you won't be too far off(at least less than if you just try your luck).
or you could buy a multimeter or simply a 75ohm adapter ^_^.