Some pools do offer such a payment method as PPS+. This method is claimed as better than the PPS method, it should insure against losses on orphans and blocks with bad luck. But this explanation is too fuzzy and obscure for me so I do not understand how to calculate exact payments. Is it true that the PPS+ is not only insure against the losses on the bad-luck blocks but also prevent to earn a surplus on the block with high luck? And I need a formula to calculate rewards on per-block basis.