Yea, those are not really precision devices and have no mechanical trim adjustments to play with. Your best bet is to just find out what the min and max counts are for each channel and use the arduino map command to convert the results to say 0-100 for each axis.
int axisX;
int axisY;
int xchannel = A0; //analog input pin wired to X channel of joystick
int ychannel = A1; //analog input pin wired to Y channel of joystick
axisX= analogRead(xchannel);
axisX = map(axisX,75,1023,0,100);
axisY= analogRead(ychannel);
axisY = map(axisY,125,1023,0,100);
The numbers in bold above would have to be found out from actual raw measurements of the min and max travel value in prior testing.
http://arduino.cc/en/Reference/Map
Good luck;
Lefty