thats not how glasses work. that's not how glasses work! That's not how glasses work! That's Not How Glasses Work! THAT'S NOT HOW GLASSES WORK!!!!!
You Don't Just Put Glasses On And Your Vision Is Spontaneously Better! You Have To Get Corrective Lenses And Frames That Fit! Glasses Are Not One Size Fits All!!!!!
Well, for general reading glasses you can just eyeball it by trying a range of readymades, but as you said, it won't just magically be the first one you try..