For this type of question you take the square root of the amount of digits the calculator has. For example:
This calculator has 8 digits so you have to take the square root of 100,000,000
b/c it has 8 zero's behind the one.
you put 1 and add however many zeros behind that one but the zeros represent the amount of digits on the calculator so
square root of 100,000,000 is 10,000 then you subtract 1
and you get 999,999. you subtract one b/c the number 0 is not considered a perfect square. Therefore, 999,999 is your answer.