okay, sheldon's gear calculator is at....
http://www.sheldonbrown.com/gears/
go there and use it.
the calculator is set to default to gain, so select gear inches and your wheel size then enter 48 for your chainring and 16 for your 'custom casette'... and you'll get your gear inches. probably somewhere around 80.
obviously that doesn't really mean a lot on it's own, but if you decidde that your gear is 10% too big (just to pull a number out of my shants) you can figure out what ratio to try (ie, 10% less would be 72 inches or 48x18)