I am not quite sure I understand the problem you have correctly. You should not have problems setting the Value property of the slider control in the handler for its load event, right after you populate its items collection.
In case I misunderstood the problem you have or in case you need additional help, please open a new support ticket and send me a running project that demonstrates the problem as well as detailed information on the problem itself.
All the best,
the Telerik team
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