Hi Phil,
Thanks for your interest in Syncfusion Products.
Please refer to the following modified sample in which the selection by rows or columns will also work by showing the sum of the selected range in tooltip while mousehovering. The pasting of a range to a selected range works fine and the tooltip is getting updated immediately with the reevaluated value on pasting.
http://files.syncfusion.com/support/samples/Grid.Windows/7.3.0.20/F80330a.zipRegards,
Rajadurai