Well if its just for protective purposes in an active design or something, I'd use 6ohm. Whether you use 6 or 7 won't matter all that much for that purpose.
But you've identified one of the biggest reasons you can't just use that simple formula for XO design. You should use a program like sound easy or PCD or something that will calculate it every 1/24th octave. Then it'll give you the true response. For instance. Using that formula you'll get something like 25uF using 6 or 7 ohms. But a 25uF cap will cause some attenuation above 500hz as well. I'm sure you already knew that, but worth saying.