[jQuery] building a custom "speed"

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I was wondering if any one could point me in the right direction for
creating a custom "speed" plugin that extends the current speeds.  For
example right now I use slideUp('slow'); .  But I want to use
something like slideUp('custom'); .  Where "custom" takes the height
of the object and computes the number of ms to return for slideUp.

ie

slideUp( (($(this).height / 470) * 1000) );

but I would like to write it like ...

slideUp('custom');

Thank You