When performing a retouch on grey hair is it best to use a formula at the true shade (7n) or 1 or 2

My client has previously colored 7N hair (which I dont have the formula for) and 90-100% grey roots. Normal porosity.

What color line/formula would you suggest?

How do I avoid getting a line of demarcation?

When performing a retouch on grey hair is it best to use a formula at the true shade (7n) or 1 or 2 lighter?

1 shade lighter

When performing a retouch on grey hair is it best to use a formula at the true shade (7n) or 1 or 2 lighter?

I wouldn't go lighter. The color will lighten on the grey hair as she washes it.