Answer here !

There are two Methods to calculate Screening constant or shielding constant

1. Experimental method : in this method if ionisation energy is given, we can apply I.E. = 13.6Zeff^2/n^2 from which Zeff can be calculated. So Zeff = Z – b, b can be calculated.

2. Theoritical method: this is an approximate method in which different orbital electrons are assigned different in screening constant.
If we have to find b for nth shell electrons
(n+1, n+2, n+3) has 0 contribution in b
nth shell e has 0.35 times their number-1 because for 1 electron, we have to find b, so all other e will contribute in b
(n-1) shell e contribution in b depends on orbital type. If e is in (n-1)d or f then its contribution is 1 times their number and if e is in (n-1)s or p then its contribution is 0.85 times their number.
(n-2, n -3) shell e contribute 1 times their number in b.
e.g. for
Zn: 1S2 2S2 2P6 3S2 3P6 4S2 3d10
b for 4s e = 0.35×1+0.85×8+1×10+1×8+1×2
For 3P e = 0×2+0.35×17+0.85×8+1×2
Sir, it is an approximate method, so there are some slight changes in contribution in b but most of the book follow this contribution rule..