APPROX

float max=10,min=0,need =3,read=0;
float x1=5,x2=0;

void setup() {Serial.begin(9600);}

void loop()
{
  read=0.7*x1+1;
  if (read<need){
    x2=(max-x1)/2+x1; min=x1; x1=x2;}
  else if (read>need){
    x2=(x1-min)/2+min; max=x1; x1=x2;}
  Serial.print("Read value :");
  Serial.print(read);
  Serial.print("  set value :");
  Serial.print(need);
  Serial.print(" control value:");
  Serial.print(x1);
  Serial.print(" max :");
  Serial.print(max);
  Serial.print(" min :");
  Serial.println(min);
  delay(1000);
}