I think most of the online calculators were accurate enough, at least (+)( -) 5% at the time when you calculate. The most important variable may global hashrate and coins price, while difficulty depends on the coin, some coin has diff-retargeting on some specific blocks, such as Bitcoin that will retarget the difficulty for every 2016 blocks solved.
Don't forget about your own condition, such as where are you mining in? Mining pool? Which mining pool? How about the pool's payment scheme?
I still using Whattomine as my fav calculator, With the right input, we will get a good result (at least).
Most have this but, I'm really here because it seems like every calculator is trying to sell you an ASIC. (Could be my ignorance).
Maybe you need to have it, lol