This can be used to re-layout a Grid with star (*) rows within a ScrollView. For Xamarin Forms

Example.cs

//usage:

		// ContactGrid is the x:Name of the Grid within the ScrollView
		protected override void OnAppearing()
		{
			base.OnAppearing();
			ContactGrid.LayoutStars(((float)App.INSTANCE.Resources["PageHeight"]));
		}
    
//In your MainActivity:


		private void InitAppResources()
		{
			App.INSTANCE.Resources["StatusBarHeight"] = GetStatusBarHeight() / Resources.DisplayMetrics.Density;
			App.INSTANCE.Resources["NavBarHeight"] = GetNavBarHeight() / Resources.DisplayMetrics.Density;
			App.INSTANCE.Resources["screenHeight"] = Resources.DisplayMetrics.HeightPixels / Resources.DisplayMetrics.Density;
			App.INSTANCE.Resources["screenWidth"] = Resources.DisplayMetrics.WidthPixels / Resources.DisplayMetrics.Density;
			App.INSTANCE.Resources["screenDensity"] = Resources.DisplayMetrics.Density;			
			App.INSTANCE.Resources["PageHeight"] = (Resources.DisplayMetrics.HeightPixels - GetStatusBarHeight() - GetNavBarHeight()) / Resources.DisplayMetrics.Density;
		}

		private int GetNavBarHeight()
		{
			int resourceId = Resources.GetIdentifier("navigation_bar_height", "dimen", "android");
			if (resourceId > 0)
			{
				return Resources.GetDimensionPixelSize(resourceId);
			}
			return 0;
		}

		private int GetStatusBarHeight()
		{
			int resourceId = Resources.GetIdentifier("status_bar_height", "dimen", "android");
			if (resourceId > 0)
			{
				return Resources.GetDimensionPixelSize(resourceId);
			}
			return 0;
		}
    
//In your AppDelegate:


        private void InitAppResources()
        {
            App.INSTANCE.Resources["statusBarHeight"] = (float)(40 / UIScreen.MainScreen.NativeScale);
            App.INSTANCE.Resources["navBarHeight"] = (float)(88 / UIScreen.MainScreen.NativeScale);
            App.INSTANCE.Resources["screenHeight"] = (float)(UIScreen.MainScreen.NativeBounds.Height / UIScreen.MainScreen.NativeScale);
            App.INSTANCE.Resources["screenWidth"] = (float)(UIScreen.MainScreen.NativeBounds.Width / UIScreen.MainScreen.NativeScale);
            App.INSTANCE.Resources["screenDensity"] = (float)UIScreen.MainScreen.NativeScale;
            App.INSTANCE.Resources["pageHeight"] = (float)((UIScreen.MainScreen.NativeBounds.Height - 40 - 88) / UIScreen.MainScreen.NativeScale);
		}

GridLayouter.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Forms;

namespace ProfiApp.Utils
{
	public static class GridLayouter
	{
		public static double LayoutStars(this Grid g, double desiredHeight, int minStarHeight = 0)
		{
			double gridMinHeight = 0, finalHeight = 0;
			int starCount = 0;
			// start with padding
			gridMinHeight += g.Padding.Top + g.Padding.Bottom;
			gridMinHeight += g.RowSpacing * g.RowDefinitions.Count - 1;
			// no look for star rows
			List<RowDefinition> starRows = new List<RowDefinition>();
			foreach(var rowDef in g.RowDefinitions)
			{
				if(rowDef.Height.IsAbsolute)
				{
					gridMinHeight += rowDef.Height.Value;
				}
				else if(rowDef.Height.IsStar)
				{
					starCount += (int)rowDef.Height.Value;
					starRows.Add(rowDef);
				}
				else if(rowDef.Height.IsAuto)
				{
					//ToDo implement Auto
				}
			}
			double starHeight = (desiredHeight - gridMinHeight) / starCount;
			if (starHeight < minStarHeight)
				starHeight = minStarHeight;
			finalHeight = gridMinHeight;
			foreach (var starDef in starRows)
			{
				starDef.Height = new GridLength(starDef.Height.Value * starHeight, GridUnitType.Absolute);
				finalHeight += starDef.Height.Value;
			}
			return finalHeight;
		}
	}
}