DLyons Apr 1, 2015:

The more theoretically correct way to do this is given in the SI brochure "Phrases such as 'percentage by mass', 'percentage by volume', or 'percentage by amount of substance' should not be used; the extra information on the quantity should instead be conveyed in the name and symbol for the quantity." That would imply stating that the units were e.g. "total kcal per day" and then just using the "%" sign e.g. "40 %" or "40 % of total kcal per day", "40 % of total kcal/day".

DLyons Apr 1, 2015:

This isn't as obvious as it might appear at first. If it were "40%" the SI brochure says " When it [%] is used, a space separates the number and the symbol %" - so that should be "40 %". But that ignores the "en" component.

If the "en" component were a unit of measure, SI is also clear "The numerical value always precedes the unit, and a space is always used to separate the unit from the number" - so "40en" should be written "40en".

But "en" isn't really a unit - it's an abbreviation for "energy". So it boils down to what sort of component is "en%" - for me, it's the abbreviated form of a non-standard unit "Energy Percentage", and my inclination is to keep the abbreviation as-is (having first defined it!).

Hence, my "40 en%". Since that's how your author expresses it, I'd stick with that.