pH value of ammonia?

Calculate the pH value of an ammonia solution that has a mass concentration of 102.18 g/L.

Given M(NH3)= 17.03 g/mol ; Ks= 5.6 ×10^-10 (mol/L)^2

My solution is:

First I calculated the Pks value

-log(Ks)=PKs= 9.25 rounded

Pkb =14-9.25= 4.75

Then I calculated the substance concentration by calculating C=ß/ M and I got 6 mol/l

Now I have POH= 1/2(4.25-log(6)= 1.735

Now I have PH=14-POH which means 14-1.735 = 12.26

Did I calculate correctly? Because the solution simply says PH=12

If so, when can you simply round the pH value or if you get a pH value of 9.84, for example, can you then say 10

Thank you very much