I just put new Z-A cells in my LunaSix and checked it against the two other meters I use regularly, a Weston Master III and a Nikkormat FTN.

Pointed at the skylight, solid bright white featureless overcast -
Gossen f:16 @ 1/200
Weston f:16 @ 1/200
N-mat f:16 @ 1/60 (*)

Pointed at a white wall, same room -
Gossen f:4 @ 1/30
Weston f:4 @ 1/40
N-mat f:3.5 @ 1/30 (*)

* the Z-A cell in the Nikkormat has been there for 37 months. I'll get my Fluke out and measure the OC voltage later on today; I think the internal resistance has risen to the point where it is no longer useful, it used to agree with the Gossen +- 1/3 stop across the range. This is the longest one has ever lasted.

The cells in the Gossen are fitted into "Cs" made of bare #12 AWG wire. 3 of the four airholes are blocked off by using gold nailpolish (just in case it's conductive) to glue three 1/16"~ squares of Al foil over the airholes of each cell. The needle goes to the far end of the red block on test, I replace the cells when it falls to the near end of the red block. They last 2+ years.