Well... first off, 100% would be... well, lard I guess?
I think you misunderstand fat content.
"whole" milk is 3.25% milk fat by volume. What I have is 1% and 18%. The sour cream is actually a relatively high fat content at 14% ("light", by comparison, is typically around 5%). Yogurt wasn't mentioned.
So with the science of it, I could do the math and combine appropriate amounts of 18% and 1% to get pretty close to a 3.25% yield... but that wasn't really the question.
As I mentioned, comments were that the recipe was too dry. As I'm sure YOU know :)... baking is just as much an art as it is a science. The classic time-tested 1-2-3-4 cake recipe has endured for (hundreds of?) years... are we really to believe that it actually happens that this radio is scientifically derived? No, its more a matter of round numbers, convenience, and easy to remember. Every recipe can be tweaked. This one, according to others, is too dry so people add "more milk" (which is vague) and 1/4 cup fo sour cream (no mention of fat content). So my question, really, was for opinions and feelings on what I might try with the 18%/1% mixture as 1) a substitute for 3.25% and b) to accomodate the "dry" aspect.
I have to run to the store now anyway... not enough butter... but I'm still hesitant to buy whole milk for this unless there's a good reason :)