The secret is an ice cream scoop (surprise!).
The scoop is used to accurately measure out cake mix into cupcake pans. A standard size ice cream scoop (with a "plus" is what I defined as a "little extra") is enough to fill your standard cupcake pans to the required 2/3 for uniform cupcakes. This trade secret will also eliminate the need for visual measurement to accurately fill each cup (and the resulting mess from leaking while pouring and eyeballing your work at the same time). It is available in graduated sizes that work for mini cupcakes or jumbo cupcakes, but choose one with a release mechanism for convenience.