Having trouble with a FOR loop.

[J-M-L]

the way your recvWithStartEndMarkers(); function works is non blocking - so you should not try to read or do anything with the receivedChars buffer until newData == true (which you clear when you call showNewData())

void loop() {
  recvWithStartEndMarkers();
  if (newData) {
     // here you have received your full message
    Serial.print("Height in Feet =  ");
    Serial.println(receivedChars);

     // handle what you need to do
     // ......

     newData = false; // get ready for the next message
  }
}