Somewhere between 20% and 40%.

Most of the imported cheap crap can be manufactured here and sold for less cost when manufactured in modern factories under sensible tax and regulation burden.

A manufacturing sector that large of solely premium products, let's say ipods and giant airplanes and really good tires, is probably not possible.

Just like everything in our lives, moderation by mixing the mundane with the excellent and everything in between is the right answer.  The consumer should be left alone to determine the mix via demand.  The manufacturer should be left alone to supply the demand and to come up with new products to separate consumers from their money.

The underlying reason for the reach to manufacturing are the historically good wages, stable employment, and working conditions that at least to start look better than juicing cows or working from an hour or two before daylight until well after dark seven days a week, 52 weeks every year, without a break.  

If our economy supplies new jobs outside of manufacturing that provide the same opportunity for workers to make similar wages, there will be no clamor to restore manufacturing.  

However, failure to keep a strong manufacturing presence in this country is folly as in the time of need, the time lag to redevelop capability and more importantly, skilled workers, is too long to meet the emergency.   Satisfying this requirement means capability, capacity, and expertise in every sector of manufacturing, not just the glamorous ones.