We use PPLNS (pay per last N shares) with a novel fee structure.
That is, a maximum of 2.5% and decreases with the hashrate contributed by the mining operator.
(see table below).
The N value used for PPLNS is the network difficulty when a block is found.

Fees are applied to each tier for each user. Fees are applied to the full block amount, plus transaction fees.
The hashrate used in the fee calculation is the hashrate recorded during the pplns window: difficulty accepted/time.
A flat rate of 0.5% is reserved for further development, with an initial focus on ckpool capability expansion.