What is th best way to go back medium blonde from platium. My hair is more white blonde. If I need I toner what is the best shade to use or can I do it with hair color.

What you're doing is not extreme. It's possible that you'll need a little warmth added if your hair is truly platinum. In that case, use a level 8 golden blonde (do a strand test first). Stay away from the ash blonde shades when going darker.

You can also try a "reverse highlight" using a level 7 or 8 dye.

By the way, demi-permanent dyes (semi-permanents that use a weak developer) are usually more permanent on bleached hair than are the so-called permanent dyes.