For Cu+, effective nuclear charge felt by a 3d-electron is

Cu(29)1s2⏞(n−3)2s22p6⏞(n−2)3s23p63d10⏞(n−1)4s1⏞n                                              ↑                                            This electron is screened by 18 electrons in (n - 1), 8 electrons in (n - 2) and 2 electrons in (n - 3). Thus, σ=8×0.85⏞(n−1)s2p6+1×10⏞(n−1)d10+8×1⏞(n−2)s2p6+2×1⏞(n−3)s2=26.8Thus, Z∗=29−26.8=2.2