Before we start with the standard junk about "imperial measures are better because they are easier to understand / better suited to human scale / etc." can I say nope . The only advantage that imperial measures have (for some) is that they are familiar. That's the sum total of it. People where were raised metric find these apologies for imperial measures to be pure gibberish.
Actually, I think metric could be improved with a few imperial equivalents - A "metric foot" as 30cm, for instance - because they are convenient measurements. e.g. If somebody asks me "How tall is that person?" it's easier to say "About five foot" rather than "I dunno, about 155cm?" (I'm British, btw)
The 30cm unit would improve absolutely nothing, it would only complicate things. If you want to use approximate measurement, just round it to tens of centimeters. Or you can use meters with one decimal place, whatever comes more natural.