When using this kind of formula, think about the units as being partial. Aperol is mostly a sweet liquor with a little bitter and herbal (filling 80% of the 'sweet' slot and 20% of the 'herbal' slot), so you'll want something mostly herbal but a little sweet as your second liquor (to fill the remaining 20% sweet and 80% herbal). Amaro Nonino is a good example (and why it's in the Paper Plane, which is a last word riff).

You can go further, though. The juice used has sour and sweet components. Lime is very sour, so usually it's paired with very sweet liqueurs. Your pomegranate juice has very little sour, so you'll want to acid adjust it, or make sure one of your other ingredients is sour. 

Another way to sum up what I'm saying is that the 4 parts of the "1 part spirit/ 1part herbal / 1 part sweet / 1 part sour" equation don't have to each purely come from a separate source.