Also worth noting, LTspice lets you parameterize component values directly from within LTspice. You can set up conditions that drive a calculated value prior to simulation. There's also quite a good Yahoo group dedicated to all things LTspice.
Want to filter a bias, reference or even supply voltage effectively, but using only circuitry that runs off a much lower voltage rail? Might sound impossible, but it’s not. The Filter Wizard shows how it’s done.