Wheel suspension, how much to lower?

[Stout001]

Today I put on new 17" wheels on my daughters cabrio. So far so good. But there is too much space between the wheel arch and the wheel, specially back. I have to buy new springs but don't know how much to lower the car. Is 30mm enough to get a nice smooth look?

Do I have to change the struts as well or is it enough to change springs?