First client Monday on the floor at school - Question about process

Hi! Super excited (and nervous) I start in the floor at school Monday. She wants a root touch up. Her roots are grey and her color is a golden blonde. So since she’s grey I know I don’t need to apply lightener, and to use 20 vol with whatever color I choose. But my question is, how do I know what color to use to match her color? I know I should use the level finder to it and then I guess select the color from there? What if it doesn’t match perfectly? TIA!