Here's what I do.
Put the line on the rod.
Don't worry if it's a size heavier, lighter, odd tapered, or whatever.
Fish with it.
If I don't like it, take it back.
I've found that I can 'figure out' how to cast any rod with any line rated between two sizes smaller and four sizes heavier than the 'rod rating' pretty easily.
I'm not a great caster. So as long as the line will take my fly to the fish, I don't sweat 'optimum' too much. For those fishermen that are really good casters, I'd think they would know exactly what they were doing and could figure it out pretty quickly. For those who are beginners, close is just fine and they'll learn with whatever line they have.
Buddy
It Just Doesn't Matter....