Program Runs Twice then Freezes

Take advantage of integer arithmetic truncation, use an array.

// start from 500
String[] values = {
  "S1", "S1", "S2", "S3", "S4", "S5",
  "S6", "S7", "S8", "S9", "S10", "S11"
};

String MathFunc(int FSRval)
{ 
  if (FSRval <= 500)
  {
    eCueValueRet = "S1";
  }
  else if (FSRval > 1050) {
    eCueValueRet = "S0";
  }
  else {
    eCueValueRet = values[ (FSRval-501)/50];
  }

  delay(50);   
}