Couple points. If you're working with raw, you're better off with an off white (non spectral white) than gray. That's due to the linear data encoding of this data. You can
use gray, it's not optimal for raw but is for a gamma corrected image. Using gray could produce some color shifts, kind of defeating the purpose.
Does the off white card need to be totally neutral? Be nice. But I find that 99 times out of 100, a WB on a neutral card doesn't appear as I desire and at least in Adobe raw converters, I have to season to taste using Tint/Temp sliders. Apparently X-rite and Warmcards agree as at least with the Passport, you have a group of off-white (cool to warm) white's to use for WB. Clever and it works real well.
So I wouldn't agonize over the neutral values unless you have a very specific kind of work to do where that would be rather critical. I've heard of people using coffee filters for WB cards with great success! But again, if starting from scratch, I'd go Passport in a second. You get a Macbeth color checker which is useful. You get the differing whites plus one big target that is rather neutral. You get a protective case. You can build DNG camera profiles with it. All for $99. Screaming deal.