Why buy panzer grey in the first place if you are to change it to medium grey?
Adding a % of white is fine but we rarely see things in white light alone. The effect can often be like your model has just gone through a chalk storm. Things often take on the hue of what is around them and so on grass a faint amount of green affects neutral colours or whites. Desert, snow scenes, urban scenes would all need a different approach to the standard %age. I prefer to use weathering powders, washes over the stock colour and references to photos of the actual things(where they exist)rather than a formula. The balance between highlight and shadow is hard to achieve and another factor in scaling. I appreciate gamers who accentuate this to make their figures/models visible on a gaming table but that is a practical requirement. The eye is the judge and jury.
I have done this (distance and scaling) with 3D computer modelling and the variables are far more than adding a bit o'white- diffuse colour, ambient colour, translucence, reflection, refraction, bump, displacement for just a few. A quick run through Photoshop also has most of the colour/distance issues in its menu.
I have a model of a Dragon Neubaufahrzeug tank to my left which is basically Panzer grey but if I ran a colour picker over it there would be umpteen variations of that original colour.