Try using a style. For instance, the following one is applied to all the rectangle instances but you could also give it a key and apply it individually to your rectangles:
<Style TargetType="Rectangle">
<Setter Property="Canvas.Left">
<Setter.Value>
<MultiBinding Converter="{StaticResource Placer}" ConverterParameter="0.5">
<Binding Path="ActualWidth" RelativeSource="{RelativeSource AncestorType={x:Type Canvas}}"/>
<Binding Path="Width" RelativeSource="{RelativeSource Self}"/>
</MultiBinding>
</Setter.Value>
</Setter>
</Style>
In order to parameterize MultiBinding.ConverterParameter
you may simply use a binding.
EDIT: I stand corrected about binding to MultiBinding.ConverterParameter
: it is not possible since it is not a DependencyProperty
but you can work around it.