Pick the desired result, choose an interpretive methodology to match, and then, time permitting, do some research to find supporting evidence.
I knew this guy who was trying to figure out the proper procedure for grilling a good hamburger. And, can you believe it, he started with a premise about what a good hamburger should be like -- slightly charred but tender, juicy, with a nicely toasted bun -- and then he designed his grilling method to reach that result! Isn't that crazy? Obviously he should have been trying to figure out, as an a priori matter, what it means to correctly grill something, and then been satisfied with whatever hamburger that method produced!
Someone theorizing about constitutional interpretation, on the other hand, is not -- or at the very least shouldn't be -- trying to determine some objectively correct, logically required method of translating constitutional text into legal rules; rather, they're trying to decide, as a normative matter, what method is going to produce the best results (whether that's understood as "best protecting people's rights," or "improving overall welfare," or "creating and sustaining a just political/legal order," or whatever).
Obviously he should have been trying to figure out, as an a priori matter, what it means to correctly grill something, and then been satisfied with whatever hamburger that method produced!
Both types of logic have their place but I find it ironic that within law there are proponents of both. I know of no examples of a similar disagreement within medicine.