The Bid price on the Post tab is usually set to 80% of the Buyout except when you use Undercutting (or when you alter the prices manually).
When you Undercut, it actually does look for the lowest bid price and undercuts that by the same percentage or amount as is used for the Buyout - sorry there is no setting to make them different.
I've always thought this was undesirable, as it means that if anyone has accidentally posted with a bid price of a few copper, SimpleAuction will try to undercut it, making Undercut a little unsafe on occasion.
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