That's only ever a guesstimate unfortunately, and the headunit output usually never reaches what they say it wil (in your case 2.5v).
What input range does your amp have (well, what amp is it?). Quite often it'll say something like 0.2 - 5V for the input level.
The best way to set a system up is to use a oscilloscope, but its not always easy to have that done!
I suggest setting your headunit to around 90% of maximum volume, playing a test tone CD (for subs, a test tone of 80Hz or so, and for normal speakers, 1KHz, the headunits quite often clip their outputs, unless its an Alpine which won't). With the amp gain set to minimum, slowly turn the gain level round (subwoofer/speakers getting louder) until you hear distortion/an unclean reproduction of the test tone. Turn the gain back down slightly and this is your rough 'max' of the system, setup to the rough 'max' that your headunit should be good for, so just don't go louder than this!
It's not a completely accurate way of doing things, but is better than just second guessing with the gain. You might want to do this in a quiet location
.
Otherwise if you can't be bothered, set it to half gain and keep your fingers crossed!
