Consider the following electrochemical cell at standard condition.

$$Au(s)|QH_2,Q|NH_4X(0.01M)||Ag^+(1M)|Ag(s)$$

$$E_{cell} = +0.4V$$

The couple $$QH_2/Q$$ represents quinhydrone electrode, the half cell reaction is given below:

[Given: $$E^o_{Ag^+/Ag} = +0.8V$$ and $$\frac{2.303RT}{F} = 0.06V$$]

The $$pK_b$$ value of the ammonium halide salt ($$NH_4X$$) used here is ______. (nearest integer)