Do you have any control over the way the data is coming into you? I think the easiest way, since the data is already coming in aggregated to a percent, is to create the other category before it ever comes into Tableau. You could request they do that calculation and union it to your data source. So that it already comes in as an "Other" category.
There is likely a way to do it in Tableau, but (perhaps it is because it is early where I am, and I'm not fully awake yet) the calculations that may be necessary in tableau to do this will be much more work then ensuring this other data comes in to begin with.