just look at pool estimate, they are correct, right now at diff 470(was 470 when i made my calculations) you produce only 430 coins per day with 40mh/s
{
"blocks" : 8606,
"difficulty" : 421.10843373,
"networkhashps" : 26503799366,
}
{
"blocks" : 8610,
"difficulty" : 323.82327395,
"networkhashps" : 25943618605,
}
This is why I disregard the difficulty and work with networkhashps.