That could be a possible approach you could take, TGI, but you could also incorporate 2 examples in each paragraph. In an expository essay, the main idea or topic sentence of each body paragraph should be a general concept. Then, you'd have 2 examples explaining this concept, and also linking back to the given prompt. Don't take the percentages too seriously though, as that'd make your writing a bit too formulaic IMO. Just ensure that, within your paragraphs, the evidence you provide from your primary text is superior to the stuff you quote from new world examples and your secondary text.
This post probably made hardly any sense, let me know if I need to clarify anything.