Magnetometer biases issue - arduino

Question

I am using lis3mdl magnetometer in my quadcopter project to compensate gyroscope drift. Unfortunatelly Im having problems probably with calibrating.
Ive achive max and min values (what is weird they are 14 bits instead of 16) and calculated biases like that :
biases[i] = (MAX_VALUES[i]+MIN_VALUES[i])/2;
(where i represent each of 3 axis).
Ive substracted biases from raw values x = (double)new_x-biases[0]; (etc), and then wanted to calculate heading like that :
heading = atan2(x,y);
heading += declinationAngle;
where declination angle is calculated.
Outcome are angles (conversion from radians heading*(180/M_PI)), and it does change when Iam rotating quad in yaw axi, BUT when I am rotating it in roll and pitch axi value change either. I want to achive stable yaw value which does not change when Iam rotating object in other axis. Maybe some type of fusing with accelerometer?
I am not sure when Ive made mistake in my calculations...
Whole class:
class Magnetometer {
int x=0,y=0,z=0;
LIS3MDL mag;
int running_min[3] = {32767, 32767, 32767}, running_max[3] = {-32768, -32768, -32768};
double mag_norm = 0.0;
double declinationAngle = 0.0;
double heading=0.0;
const int MAX_VALUES[3] = {3014,3439,10246};
const int MIN_VALUES[3] = {-4746, -4110, 492};
double biases[3] = {0.0};
double scales[3] = {0.0};
double avg_scale = 0.0;
ButterworthDLPF xyz_filter[3];
double DLPF_ON = true;
const float sample_rate = MAG_SAMPLE_RATE;
const float cutoff_freq = 4.0;
public:
Magnetometer() {}
void Init() {
declinationAngle = (6.0 + (8.0 / 60.0)) / (180.0 / M_PI);
for(int i=0; i<3; i++) {
biases[i] = (MAX_VALUES[i]+MIN_VALUES[i])/2;
scales[i] = (MAX_VALUES[i]-MIN_VALUES[i])/2;
}
avg_scale = (scales[0]+scales[1]+scales[2])/3.0;
for(int i=0; i<3; i++) scales[i] = avg_scale / scales[i];
Serial.println("Turning on magnetometer. . .");
if(!mag.init()) {
Serial.println("Failed to detect magnetometer!");
ESP.restart();
}
mag.enableDefault();
//Calibrate();
for(int i=0; i<3; i++) xyz_filter[i].Init(sample_rate, cutoff_freq);
Serial.println("9DOF readdy!");
}
void Calibrate() {
delay(100);
while(true) {
mag.read();
if(running_max[0]<mag.m.x) running_max[0] = mag.m.x;
if(running_max[1]<mag.m.y) running_max[1] = mag.m.y;
if(running_max[2]<mag.m.z) running_max[2] = mag.m.z;
if(running_min[0]>mag.m.x) running_min[0] = mag.m.x;
if(running_min[1]>mag.m.y) running_min[1] = mag.m.y;
if(running_min[2]>mag.m.z) running_min[2] = mag.m.z;
Serial.println((String)running_max[0]+" "+(String)running_max[1]+" "+(String)running_max[2]+ " "+(String)running_min[0] +" "+(String)running_min[1]+" "+(String)running_min[2]);
delay(20);
}
}
void Update(){
mag.read();
xyz_filter[0].Update(mag.m.x);
xyz_filter[1].Update(mag.m.y);
xyz_filter[2].Update(mag.m.z);
//Serial.println(xyz_filter[0].getData());
/*x = ((double)xyz_filter[0].getData()-biases[0])*scales[0];
y = ((double)xyz_filter[1].getData()-biases[1])*scales[1];
z = ((double)xyz_filter[2].getData()-biases[2])*scales[2];*/
x = ((double)mag.m.x-biases[0])*scales[0];
y = ((double)mag.m.y-biases[1])*scales[1];
z = ((double)mag.m.z-biases[2])*scales[2];
CalculateHeading();
}
void CalculateHeading() {
heading = atan2(y,x);
heading += declinationAngle;
//if(heading<0) heading += 2*PI;
//else if(heading>2*PI) heading -= 2*PI;
heading=MOD(heading*(180/M_PI));
}
double GetHeading() {return heading;}
void ShowRawValues(bool names=false) {
if(names) Serial.print("X: "+(String)x+" Y: "+ (String)y+ " Z: " + (String)z);
else Serial.print((String)x+" "+ (String)y+ " " + (String)z);
}
};