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);
}