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.
That's not exactly correct, the p2p pools mine on a different chain that is shared amongst all the miners. This chain tends to move at a faster rate than the normal block chain, hence the higher rejects that some people report. P2p pools usually are able to report found blocks faster than a non p2p pools which gives them a slight edge over a normal pool. You will want to make sure that your connection to the pool is good (at one location I averaged about 2-3% rejected/orphaned on free.btc:9173, but another location (behind university firewall) didn't seem to like to maintain a stable connection and caused only about 2/3-3/4 of my actual hashrate to be reported...)).