How to find SHA1 output having 'zero' in first 8 places

I have to find SHA1 output having 'zero' in first 8 places.

Output[40] = SHA1( Data[64] + Random[??] )

I cant figureout equation for generating random string to find required SHA1 in minimum iterations.

Random Generator

String GenRand()
{
    String possibleCharacters("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"); // change if require
    int randomStringLength = 12; //8 // change if require 

   String randomString;
   for(int i=0; i<randomStringLength; ++i)
   {
       int index = rand() % possibleCharacters.length();
       Char nextChar = possibleCharacters.at(index);
       randomString.append(nextChar);
   }
   return randomString;
}

Data[64]:

HOPETDtHmRghhwLNEdYJNKYSRdUIkoKAxfhaQQTtcSrKrlgjpQBRKAXyANYqtFFP

Random[??]:

awtVstgafWfa

Required Output[40]:

000000006259aa374ab4e1bb03074b6ec672cf99