You should get your hashrate in kh/s divided by the diff in coins/day. But luck plays a big part too. The smaller the pool your mining at the more luck factors in. With a p2pool you also get a ramp-up/ramp-down in rewards/block and you will need to gather data over a longer period than 24hrs.
And generally p2p pools have 10-20% Orphaned or DOA shares. IE LOST HASH.
DeltaQuebec did explain it but but to be more specific about the 'LOST HASH';
The doa% for the pool IS much higher with p2pool. But that doa% is for the p2pool blockchain and does not propagate outside to the normal blockchain. And thus will not influence payouts. The doa% for your miner is also a little higher and you will get a lower payout if you don't try to get that % down as far as you can (best by choosing a node with a low ping time and lowering the intensity on your software).
The blockchain for p2pool is 10? times faster than the normal blockchain, and also has a 10? times lower difficulty. This will result in the higher doa%. When a block is found within the p2pool that block often propagates faster to the outside world, making the chance it gets orphaned smaller. As with all pools in p2pool the rewards are shared among the miners. Your share% depends on your hashrate-doa so try to get your doa% lower. The shares that are orphaned from your p2pool node to the other p2pool nodes do not matter. They are not at a sufficient difficulty for a 'block found' anyway, and i think they will still count as valid shares in your node (not sure about that though).