Most American measuring cups don't use decimals, unfortunately. You get things like ¼, ½, and ⅓ cup measures. Of course, still doable, just takes some extra mental work.

And if you can do that much mental work, you probably don't need the slide rule. I think a slide rule graduated with "cooking fractions" (i. e. the sizes of measuring cups and spoons that people actually have) would be a good idea.