... For larger miners (over 10Gh), getting a 5 btc payout twice a day does not work out at my pool balance would just keep rising. ...
... There are only two payouts per day, so set the threshold to 0.2. You will get the highest multiple of 0.2 as the payout. ...
Fred0 describes it well.
I've also just changed the wording on the account page to make it clearer how auto-payout is done.