How to get scale factor of viewbox?

I have some problem: need to make animation that starts from element inside viewbox and finishes outside viewbox

It means:

I have page where elements are placed. There is text block on the top of it and viewbox. I place gridview into this viewbox. And one moment it should start animation from gridview item

 <Viewbox HorizontalAlignment="Left" Stretch="Uniform" VerticalAlignment="Top" x:Name="vbChallengeView" >
                    <Grid x:Name="gdChallenges">
                    <GridView  
                        Margin="92,64,68,0"
                        x:Name="GvChallenges"
                        ItemsSource="{Binding Source={StaticResource GroupedItemsViewSource}}"
                        VerticalAlignment="Top"
                        HorizontalAlignment="Left"
                        ItemTemplate="{StaticResource ChallengeTileTemplate}"
                        ItemClick="GvPack_OnItemClick"
                        IsItemClickEnabled="True"
                        IsZoomedInView="False"
                        SelectionMode="None"
                        >
                        <GridView.ItemContainerStyle>
                            <Style TargetType="GridViewItem">
                                <Setter Property="IsTabStop" Value="False" />
                               <Setter Property="Margin" Value="0,0,24,24" />
                            </Style>
                        </GridView.ItemContainerStyle>
                        <GridView.ItemsPanel>
                            <ItemsPanelTemplate>
                                <WrapGrid MaximumRowsOrColumns="5" Orientation="Horizontal" />
                            </ItemsPanelTemplate>
                        </GridView.ItemsPanel>
                    </GridView>   
                    </Grid>
                
                </Viewbox>

How can I determine item size in view box and it's position relatively current window?

Thanks for any ideas