Write the electron configuration and group the electrons according to Slater's rules.

The electron configuration for Potassium (K) is $1s^2 2s^2 2p^6 3s^2 3p^6 4s^1$.
Grouping for Slater's rules:

$$(1s^2) (2s^2 2p^6) (3s^2 3p^6) (4s^1)$$

The rules for calculating S for an (ns, np) electron are:

• Electrons in the same (ns, np) group contribute 0.35 each.
• Electrons in the (n-1) shell contribute 0.85 each.
• Electrons in the (n-2) or lower shells contribute 1.00 each.

For the 4s electron:

• Electrons in the same (4s) group: There are no other electrons in the 4s subshell. Contribution = $0 \times 0.35 = 0$
• Electrons in the (n-1) shell (3rd shell, $3s^2 3p^6$): There are $2+6=8$ electrons. Contribution = $8 \times 0.85 = 6.80$
• Electrons in the (n-2) shell (2nd shell, $2s^2 2p^6$): There are $2+6=8$ electrons. Contribution = $8 \times 1.00 = 8.00$
• Electrons in the (n-3) shell (1st shell, $1s^2$): There are 2 electrons. Contribution = $2 \times 1.00 = 2.00$

Total shielding constant (S):

$$S = 0 + 6.80 + 8.00 + 2.00 = 16.80$$

The formula for effective nuclear charge is $Z_{eff} = Z - S$.
For Potassium, the atomic number ($Z$) is 19.

$$Z_{eff} = 19 - 16.80$$ $$Z_{eff} = 2.20$$

The effective nuclear charge for an electron in the 4s1 subshell of Potassium is $\boxed{\text{2.20}}$.