I just bought a new stepper motor (400 steps for revolution, 0.9°/step) and I would like make a turn of 180° by 200 steps, but it doesn't work as expected. It looks like the motor needs only 110 steps to make a turn of 180° and I don't know the cause. The driver motors is the Keyes L298. It is wired to my Arduino UNO rev3.

In order to clarify, please see the image below:

The code is as follows:

#define negativeSteps -55
#define positiveSteps 55
#define Speed 7500

int motorPin1 = 11;
int motorPin2 = 10;
int motorPin3 = 9;
int motorPin4 = 8;         

int steps;
unsigned int tmp2;

byte modeR[] = {B00001001, B00001010, B00000110, B00000101};
byte modeF[] = {B00000101, B00000110, B00001010, B00001001};

void setup() {

  Serial.begin(115200); //Opens serial connection at 115200 baud.     

  pinMode(11,OUTPUT);
  pinMode(10,OUTPUT);
  pinMode(9,OUTPUT);
  pinMode(8,OUTPUT); 

  pinMode(3,OUTPUT);    // Enable pin motorA
  pinMode(2,OUTPUT);    // Enable pin motorB
  
  digitalWrite(2,HIGH);
  digitalWrite(3,HIGH); 
}

void loop()
{
    while (Serial.available())
    {
      char cmd = (char)Serial.read();
      if (cmd == 's')   while(1) Seek();
    }
}
void motorR(){
  int bitMask=B00001111; // bitmask to modify only last 4 bit
          
          for (int id = 0; id <4; id++) {
            PORTB = (PORTB &~bitMask) |  modeR[id]; 
            while (tmp2++<Speed);
            tmp2=0;
          }
}
void motorF(){
  int bitMask=B00001111; // bitmask to modify only last 4 bit
          
          for (int id = 0; id <4; id++) {
            PORTB = (PORTB &~bitMask) |  modeF[id]; 
            while (tmp2++<Speed);
            tmp2=0;
          }
}
void Seek() {
  steps = negativeSteps;      //Init Steps
  while (1)
  {
     
    while (steps<positiveSteps) //  starts from "negative" steps to "positive" steps ( -55 to +55)
    {
       if (steps<0) motorF(); // if steps = 0 it will change the direction
       else motorR();

      Serial.println(steps);
      steps++;  
   }
 }  
}