Example 1:

<UserControl.Resources>
<Elcom:Addicter x:Key="Adder"/>
</UserControl.Resources>

...

<Popup PopupAnimation="Slide" StaysOpen="False" Placement="Relative" PlacementTarget="{Binding ElementName=textblock1, Mode=OneWay}" x:Name="Popup1" VerticalOffset="-300" Height="290"
Width="{Binding ActualWidth, ConverterParameter=19, Converter={StaticResource Adder}, ElementName=textblock1, Mode=OneWay}">


<!-- Popup1.Width = textblock1.ActualWidth+19 -->

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Example 2

<UserControl.Resources>
<Elcom:Addicter x:Key="Adder"/>
</UserControl.Resources>

...
<TextBox
...
IsEnabled="{Binding IsCreated, ElementName=IVM, BindsDirectlyToSource=True, Mode=OneWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource Adder}}"/>

<!-- TextBox.IsEnabled = ! IVM.IsCreated -->