Can you share why you need to manually compute these values? By default the DataContext that is received in the GanttDragResizeVisualCueTemplate allows you to access them and you should be able to utilize them. You can access them by binding to the DestinationSlot.Start and DestinationSlot.End property of the DataContext.
If such an approach is not suitable for your scenario, can you elaborate on it so that I can get a better understanding of it and further assist you?
I am looking forward to your reply.
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