to figure out how many BB you want at final table just take the starting stack and multiply it by the number of players that entered, then take that number and divide that by 9 (number of players at final table) and that is the number of chips you will want to have.
So for example if starting stack is 1000 chips and the tourney had 700 players, you take 1000 and multiply that by 700 which equals 700,000 chips in tourney. now take the 700,000 total chips in play and divide that by 9 final table players which comes to 77,777 chips you want to have by the time you get to the final table.