Actually, it’s a bit more complicated than that, I’m not sure if I posted the formula but it’s here.
Chance of getting Unique = (1 - (0.98) exp N tries.) x 100 (for percent).
So try 1
1 - 0.98 = 0.02 x 100 = 2%
10 tries
1 - 0.82 (rounded up) = 0.18 x 100 = 18%
So 1 - (0.98 exp N) = 0.5, solving for N will give you the tries.