Wella 100% grey hair to red, help

[Kayls]

Hi just looking for a advice on colour, I've a model who's roots are 100% white and usually gets her hair done using wella 5/0 and 5/5 with 6%, wella shade chart advices to use 2parts red to 1 part base on hair with more than 50% white but weary weather it will stick only using 1 part base ? could anyone help me by explaining how it works please ! Xx