Black Scholes formula

Sir I know u said u don’t have the BPP study text but in the back of it we have a question with d1 being 1.998 and d2 being 1.898.

Now 1.9 corresponds to 0.4767 and 1.8 corresponds to 0.4706 on the ND tables.

Now in the books answer Nd1 is calculated as 0.5+0.4767+( (82/100) x (0.4772-0.4767) )
[0.4772 being the corresponding value for 2.0 on the ND table]

Now I know why we add 0.5 but whats the latter addition for?