Simple sketch to increment and decrement a single NeoPixel on a string as to determine positions of interest for defining segments.

Arduino NeoPixel Increamenter Tester

#include <Adafruit_NeoPixel.h>
#ifdef __AVR__
 #include <avr/power.h>
#endif

#define PIN        6 
#define NUMPIXELS 500

Adafruit_NeoPixel pixels(NUMPIXELS, PIN, NEO_GRB + NEO_KHZ800);
int pos = 0;

void setup() {
  Serial.begin(9600);
  Serial.println(F("Started - Simple NeoPixel increamenter"));
  Serial.println(F("(1 based Index)"));
  Serial.println(F("'-' to decreament"));
  Serial.println(F("'+' to increament"));

  pixels.begin(); 
  pixels.clear(); 
  pixels.setPixelColor(pos, pixels.Color(128, 128, 128));
  pixels.show();
}

void loop() {

  if(Serial.available()) {
    while (Serial.available()) {
      byte inByte = Serial.read(); // get command from serial input
  
      if(inByte == '-') {
        if(pos > 0 ) {
          pos--;
        }
        Serial.print(F("decreamenting pos = "));
        Serial.println(pos + 1);
      } else if(inByte == '+') {
        if(pos < NUMPIXELS ) {
          pos++;
        }
        Serial.print(F("increamenting pos = "));
        Serial.println(pos + 1);
      }
    }
    pixels.clear(); 
    pixels.setPixelColor(pos, pixels.Color(128, 128, 128));
    pixels.show();
  }
}