You can use the format string "P" to format as percentage - which by default rounds to two decimal digits. I didn't quite follow how the example of 1245 percent becoming 12 percent correlates with your statement that you want to round to the nearest hundredth of a percentage.
It should be =sum(Fields!Sales.Value, 2)/Sum(Fields!Sales.Value,"DataSet1") and now you can select % from the toolbar menu, so it will be 12.45% and if you do not want the decimals then make it 0 decimals. So the end result you would have would be 12.
======================= Not an Expert, Just a learner. !_(M)_!