Paynes grey isn't suitable for grey scale because it is a colour and not a pure grey or neutral grey, adding white or black to woukd likely give you a shade very far from what your after.
I thought you had a greyscale set delivered?, if you want to work greyscale this is what you should work with and don't use anything outside of that set.
If you go to Google images you can search for a grey scale value finder which you can download and print off, but
@haasje dutchairbrush did a very in depth how to in grey scale whuchvi sure he'll guide you to, I believe he posted a downloadable grey scale finder in that same thread.
Also, on YouTube do a search for airbrushing videos concerning cokiyr capping which will give you an idea of how to proceed in miniscule using your greyscale set.
I should point out that working with greyscale value sets is far more complicated than simply painting with an over reduced black or paynes grey and building up your values, but is important to avoid any other cokours including white, anything white should be your substrate, and when you build up yiyr values you care merely hiding parts if your substrate to different degrees.
I'm assuming you got the greyscale set hoping it would be easier than working with colour? I think you may now be finding out that that just isn't so