Meter box with float values! using: keypad + Nokia + Nema 23

Hello everyone i need this, maybe is easy for you, but i dont have a lot of knowledge with code. (i dont speek english)

The thing that i want to do is:
Read from a Keypad, some numbers and i will show them into a LCD. i want to create a numer like this:

23.4 cm

I know my code is working, but just creating a number like this: 2345 cm.

The question is.... how can I change my code for take that number with a dot, because i will work between thee range: 0,0 - 250,0 (cm) ......i really dont know what to do

The code:

void checknumber(float x){
  if (firstnumber == 99) { // Check if this is the first number entered
    firstnumber=x;
    String displayvalue = String(firstnumber);  //  Transform int to a string for display
    drawnokiascreen(displayvalue); // Redraw Nokia lcd
    
  } else {
    if (secondnumber == 99) {  // Check if it's the second number entered
      secondnumber=x;
      String displayvalue = (String(firstnumber) + String(secondnumber));
      drawnokiascreen(displayvalue);
 
    } else {  
     if (thirdnumber == 99) {  // Check if it's the third number entered
      thirdnumber=x;
      String displayvalue = (String(firstnumber) + String(secondnumber) + String(thirdnumber));
      drawnokiascreen(displayvalue);

       } else {  // It must be the 4rd number entered
      fourthnumber=x;
      String displayvalue = (String(firstnumber) + String(secondnumber) + String(thirdnumber)+ String(fourthnumber));
      drawnokiascreen(displayvalue);
 
    }
  }
}
        Serial.println(keyfullnumber);
}