How to find closest point on plane?!?

Hi guys!!

I've a problem...I want to find closest point on a plane!

I've tryed to used "Surface CP" but it doesn't work...

You can see in the image 1 below, the wrong solution and the distance is not 20 but 20.024984

Instead, if I create a simple surface on plane, it works...so you can see the distance is 20!

I don't want to create this surface but find a simple solution...do you have any idea guys?

Thanks! ;)