I think I understand where the confusion comes from, here are some factors affecting the profitability in FIAT in different calculators.
1-Bitcoin price ( The way they get it, from where, when, how)
2-Block fees estimates ( how they estimate something that is never fixed)
3-Pool fees ( Pools have different fees, which one they pick)
We know that profitability BTC wise is the same for every 2016 blocks, it does not matter what price that is, it does not even matter if the hash rate has gone up 100 times during that 2016 period, so if happens that you find two calculators which give you two different results in fiat, that's completely normal, but any difference in the profitability BITCOIN wise - then one of them is FOUNDEMANTLY wrong, and I have yet to see that.
Here an example
hashrate :1000 TH
Power usage = 0
Pool fees = 0
Block fees = 0
Calculator - Profit 24hrs
www.cryptocompare.com 0.01807
www.coinwarz.com 0.01807415
www.whattomine.com 0.018074
They all show the EXACT same results, except for that the first and the last one display fewer digits, however, if you were to use www.whattomine.com default page where it has all other values entered, they actually estimate block rewards of 12.65 BTC and average block time of 9m 54s based on whatever "internal" calculations they come up with, probably taking the current hash rate into account and the fact that philipma1957 mentioned here.
Also, bitcoin price is subjective, some might use the current price at CMC, some may use Binance alone, some may use the average of 20 different exchanges, some may use the average hourly price and etc.
Also, some calculators have a certain pool fee by default, so all those factors combined together can very well cause the 15-20% you talk about, but all of those factors ignored, you will always get the same exact result if you apply the formula posted above by mocacinno.
Now, if you want to provide the easy to read profit in $ ( I assume most people want to see that as opposed to anything else) I suggest you follow www.whattomine.com way of doing it.
Their average price of bitcoin seems more averagely accurate than many other calculators, their block fees estimates are not that bad, there are a lot of things that I would love to see changed in the calculator, but since you are not trying to develop one, there is no point in stating them here, but I still suggest you follow their way of calculating profitability as I personally find it the most reasonable.