Scrolling Grid Relative Sizes

I have managed to create a GridView inside a ScrollView that uses 8 rows of row height 250 (Absolute). This was fine until I realised it would not work on screens with different resolutions. How can I create a grid view where there are 8 rows, but each row takes up half the screen so you have to scroll down to see the rest? Using 8 rows of height "*" just puts 8 rows onto the view with no scrolling. I am using xaml but can use c# if necessary.