[Bibio]

i've done this a few times now and i have discovered that you dont even need the bike running. its a Throttle Position Sensor.

get the line in the middle.
turn the throttle to take up the slack in the cable.
turn the throttle so the line moves with the slightest turn of the throttle.. or turn the sensor till it just moves the line upwards then back off a hair. check by turning the throttle that the line moves up just as the throttle is turned.

if you want to do it with the engine running then go ahead. your looking for the line moving upwards with the slightest of revs. remember to turn the bars side to side after making the adjustment to check its still ok.

the only downside of this adjustment is that there is a lot less engine braking.

if the line is in the middle your pretty much good to go but a small tweak will make a difference at things like roundabouts etc.etc.