Read the F1 help for Set Analysis and you'll see how much is possible!! For your specific solution, try:
The first of these will divide the Sum(Volume) for all the current selections by the TOTAL Sum(Volume) when ignoring ALL selections, the second will only ignore the Widget selection.
Hope this helps,
You can do that.
Below expression won't consider selection(divide by all the widgets) to divide
Below is to get percentage in the selection
Multiply with 100 which gives you percentage or Check the Show in percentage option in the number tab