Thank you for contacting us.
It looks like you are looking for the RelativeSize
feature of the RadDocking control. In order to achieve a relative sizing of the control's elements all you need to do is set the ProportionalStackPanel.RelativeSize attached property to the RadSplitContainer instances. You can take a look this article
from our online documentation which goes if further details.
Hope this is helpful, if you have any other questions feel free to write to us again.
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