I've tried this and the motors start to move as soon as power is applied with zero stick movement
Oops sorry
In getJoystickState(), please replace
if(joyY >-10) { // differential steering
servo_L = (joyY - (3*joyX)/4);
servo_R = (joyY + (3*joyX)/4);
} else {
servo_L = (joyY + (3*joyX)/4);
servo_R = (joyY - (3*joyX)/4);
}
by
if(joyY >-10) { // differential steering
servo_L = (joyY - (3*joyX)/4) * 1.2;
servo_R = (joyY + (3*joyX)/4) * 1.2;
} else {
servo_L = (joyY + (3*joyX)/4) * 1.2;
servo_R = (joyY - (3*joyX)/4) * 1.2;
}