I can't see how those '
lens covers' can work properly to be honest .. I think the assumption is that you and/or the camera are within the lighting arrangement that is going to cause the issue.
Unless you want the grey card to help you determine
exposure, the percentage rating is irrelevant.
The 18% rating is the shade of grey that will give you a neutral RGB rendering(128, 128, 128)
Most grey cards will be 18%, as this is the most common. Handy device for determining
exposure if using film(and a
light meter is some kind).
If determining an accurate WB is you objective then a grey card is a grey card, as long as it is grey(and not tainted or stained with other markings and so forth)
Note you can also use a piece of (very)white paper, as long as the white paper is clean.
CFL lighting is difficult to balance properly, but mixed lighting is also a challenge to get colour balance right.
Mix CFLs and standard fluoros and daytime ambient, and you have a hard time finding a good balance.
Are these Grey Cards neccesary .. nope!
Handy .... yes for sure ... but not necessary.
My large fold up grey card thing cost about $20. Handy when it's needed, but it's not needed that often.
It folds in on itself down from 56cm to about 20 or 15cm and is easily transported in most camera bags .. so I thought it was worthwhile having.
And I've used it on many occasions too.