Set height of a layout depending on screen size

Hi,

On every page of my project I put an AbsoluteLayout (A) (I use it as a custom navigation bar) and I set its properties like this:

<AbsoluteLayout
                    AbsoluteLayout.LayoutBounds="0, 0, 1, 44"
                    AbsoluteLayout.LayoutFlags="PositionProportional,WidthProportional">

Below it I put another AbsoluteLayout (B) which is my actually my real page.

How can I set B's properties to be like:

<AbsoluteLayout
                    AbsoluteLayout.LayoutBounds="0, 0, 1, ScreenSize - 44"
                    AbsoluteLayout.LayoutFlags="PositionProportional,WidthProportional">

Thanks