Take the total hashing power of the EU pool and figure out what fraction of that you are contributing. That will be the maximum return you can get per block after it is split up from the 50btc. You were climbing up to that percent but cannot go above that without adding more hashing power therefore increasing the percent you are contributing.
thanks for the concise and clear explanation!
however that means something is off with my calculations: the bitcoin mining calculator shows .56 btc per 24 hours of 240mh/s, whereas on the pool it's only .36, which is a big enough difference to question rentability...