Image size, Scale or Request ?

Hi,
I have an image which I want to be a certain size relative to the screen size so it will always occupy the same amount (proportionally) of space on screen such as 0.5 the screen width. I can do this with Scale="0.5" which works fine but Scale does not adjust the bounding box.
I also have some text to display directly underneath the image using a Label. This is fine when Scale="1" but there is unwanted space when Scale="0.5" or smaller. How do I get rid of this unwanted space ?
Should I use WidthRequest instead, in which case how do I determine 1/2 the screen width ?

Cheers,
Wayne