Problems with percentage height

Hello, I am trying to make a modal 100% of height, or 90% but I cannot get it working well.

I understand that if I put `width: 90%` I have to put aswell `margin-left: -45%` but with height is not working well, at fullscreen it is very high and if I resize the window horizontally making it smaller the modal is going down.

Is there any styles to do this?