You might want to get a copy of Lee's book "Modern Reloading". If memory serves me, he devotes a whole chapter to the shooting of light loads, and even includes a small spreadsheet formula that will let you calculate various reduced powder charges. I haven't tried it myself, but it might be worth a look see for you.
Regards - AZFred
[This message has been edited by Fred (edited May 10, 1999).]