Summing a column of expressions in SSRS
Try Sum((Fields!one.Value / Fields!two.Value)).
You could add a calculated field in your data set that creates the ratio.
I - Insert the data field instead of the expression directly in the field.
Now you have it total by right-clicking and selecting add a total.
II - Right-click on your data set, select Add calculated field, name it ratio, add your expression here.
Then ratio will be a selection you can add to your table as an actual field instead of an expression. You can then total it.