Arduino Nano OLED 1.3 inch u8g lib Ultrasonic Sensor Water Level Simple

#include <Arduino.h>
#include <U8g2lib.h>

#ifdef U8X8_HAVE_HW_SPI
#include <SPI.h>
#endif
#ifdef U8X8_HAVE_HW_I2C
#include <Wire.h>
#endif


const int trigPin = 9;
const int echoPin = 10;

// Tank height in cm
const int TANK_HEIGHT_CM = 30;

// Variables
long duration;
int distance;
int waterLevel;
int percentFull;
String str;

U8G2_SH1106_128X64_NONAME_F_HW_I2C u8g2(U8G2_R0, /* reset=*/U8X8_PIN_NONE);
void setup(void) {

  u8g2.begin();
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  Serial.begin(9600);
}
void loop(void) {

  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);


  duration = pulseIn(echoPin, HIGH);
  distance = duration * 0.034 / 2;
  waterLevel = TANK_HEIGHT_CM - (distance + 5);  //5cm for the gap between the water (when full) and the ultrasonic sensor

  waterLevel = waterLevel + 10;


  percentFull = (waterLevel * 100) / TANK_HEIGHT_CM;



  if (waterLevel < 1) {
    waterLevel = 0;
  }

  if (percentFull < 1) {
    percentFull = 0;
  }

  str = String(percentFull) + "%";
Serial.println(percentFull);
  u8g2.clearBuffer();                   // clear the internal memory
  u8g2.setFont(u8g2_font_ncenB08_tr);   // choose a suitable font
  u8g2.drawStr(0, 10, "Water Level:");  // write something to the internal memory
  u8g2.setFont(u8g2_font_ncenB18_tr);   // choose a suitable font
  u8g2.drawStr(40, 40, str.c_str());     // write something to the internal memory
  u8g2.sendBuffer();                    // transfer internal memory to the display
  delay(1000);
}