setting RWIGS for a slab with vacuum

[rpnabar]

VASP Manual speaking about RWIGS:
"The sum of the volume of the spheres around each atom should be the same as the total volume of the cell (assuming that you do not have a vacuum region within your cell). "

I am doing a Pt(111) slab calculation with vacuum.
What do I do now that I DO have a vacuum spacing.

Option 1: set RWIGS close to covalent radius(also close to default in POTCAR) That gives me only a 22% volume occupancy(obvious since rest of my cell is vacuum!)

Option 2: Use dispproportainately large RWIGS spheres so that I can get close to 100%. But then my RWIGS is far off from the atomic radius.

I guess there's a tradeoff but what do people think is the best way of dealing with this?

Other settings for my projected DOS calculations:
Ultrasoft PP; GGA
LORBIT=1;
RWIGS=1.28 Angstroms

[admin]

of course that space-filling argument only holds for bulk calculations.
please choose RWIGS either as given in POTCAR, or, if you have a structure which leads to bond lengths that significantly differ from the respectives' atoms in their bulk phases, choose RWIGS according to the (weighted) covalent radii.