stiwagon

hi guys
last week i bought an sti wagon number 100 of 200 its an import on an l plate.inside the car there is an apexi afc it has 5 ***** you can turn.currently they are at 0 but you can turn them - or you can turn them up to +20.there is no manual with it so i havent got a clue what it should be at or how to work the thing.has anyone have this fitted?and could you please tell me what it should be set at or what it would be good for my car to be at for performance!
i have a full stainless steel system induction kit and also a boost valve fitted but thats currently turned off if thats any help.
also it costs me £30 for a full tank but i get 100 miles to the tank if driven hard but probably around 140 150 if driven easy is this normal for this type of car or do you think something is wrong??
id really appreciate any advice at all you can give
cheers