If you look at the info on a Masterful writing necessities scroll it says "Modifies your Craft skills by 12 point(s) for two min and 0 sec." I just took that as being true, and found it to be consistent with my success rates. :)
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Remembering Tyneetryk
Phaedreas Tears - 15 years old and first(*) of true neutral guilds in Atys.
(*) This statement is contested, but we are certainly the longest lasting.
<clowns | me & you | jokers>