I am not quite sure if I understand the exact problem. If the issue is connected with making RadSplitter occupy the 100% of the page in height and width, than you could find useful You could find useful information about how to achieve that in the following help article: Filling the Entire Page.
Please, note also that in order to receive any consistence behavior of RadSplitter it must contains at least two RadPanes.
If non of the above is the issue, could you please describe in more details the exact scenario, what is the expected and what is the actual behavior?
For your convenience I prepared a sample project demonstrating the basic way to configure a RadSplitter with height set in percents. Could you, please, modify it to the point where the problem occurs and send it back?
the Telerik team
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