Help irrigation scheduled

Hello. For some time working on a project with Arduino measuring moisture in the soil, which involves a humidity sensor placed on the pin (A0) and a Nokia5110 LCD that shows the percentage of zero to one hundred soil moisture. All want this project to also add a solenoid valve, scheduled to irrigate, put on digital pin = 4. My question is, what to write more code to open the solenoid it reaches 15% up to 100% percent, and stay closed from 100% to 15%. I’m beginner with programming help me. This is the code I have.

#include <LCD5110_Graph.h> // THE LIBRARY I AM USING IS THIS: LCD5110_Graph - Rinky-Dink Electronics

LCD5110 lcd(8,9,10,12,11);

extern unsigned char BigNumbers;
extern uint8_t ui;

int sensorPin = A0;
int sensorValue = 0;
int percent = 0;
String percentString =“0”;
int stringLength = 0;

void setup() {
lcd.InitLCD();
lcd.setFont(BigNumbers);
delay(1000);
}

void loop() {
lcd.clrScr();
lcd.drawBitmap(0, 0, ui, 84, 48);
sensorValue = analogRead(sensorPin);
percent = convertToPercent(sensorValue);
percentString = String(percent);
stringLength = percentString.length();
displayPercent(stringLength);

lcd.update();
delay(1000);
}

int convertToPercent(int value)
{
int percentValue = 0;
percentValue = map(value, 1023, 350, 0, 100);
if(percentValue>100)
percentValue = 100;
return percentValue;
}

void displayPercent(int length)
{
switch(length)
{
case 1: lcd.print(percentString,38,19); break;
case 2: lcd.print(percentString,24,19); break;
case 3: lcd.print(percentString,10,19); break;
default: lcd.print(percentString,0,19); break;
}
}

Let me understand well

You go and copy line for line an example from the Internet and you want them someone to add code into that example for you?

What work did you do “for some time” as you say ???