Fermi energy changes with the value of parameter NC_K

[ZhuoyangXie]

Dear developers of VASPsol,
I am doing some research about the explicit/implicit solvent model with VASPsol code, in which I build a slab with 1 or 2 layers of explicit ice-like water above it. I have tried to modify the parameter NC_K with the purpose to make sure the counter-ion would not seep into the explicit molecules region. However, I find that the Fermi energy will dramatically change with the variation of the value of NC_K, as follows:
e.g (NC_K, E-fermi, Vaccum level)
(2.5E-5, -5.7899, -0.368)
(2.5E-4, -5.4576, -0.369)
(2.5E-6, -4.9104, -0.368)
This is a puzzle, I wonder if the variation of Fermi energy is reasonable and how can I obtain the correct PZC by adjusting NC_K