The spread is real and it comes from what each calculator assumes about spikes.
Average draw is not what a power supply has to survive. Modern graphics cards in particular pull very short bursts far above their rated figure: brief spikes of double the sustained number are normal. Those last microseconds, so they barely register in a power meter and they absolutely can trip a supply's protection and shut the machine off. A calculator that only sums rated figures gives you the low number; one that allows headroom for transients gives you the high one.
So the sizing method:
Add the sustained figures for your components: the card and the processor are essentially all of it.
Add generous headroom on top, not a token amount. Whatever the card manufacturer recommends is a reasonable floor and they set it with transients in mind.
Buy the higher of that and your own arithmetic.
Two things that are not reasons to go bigger: future upgrades you may never make, and the idea that a bigger supply is inherently safer. And one thing that is: a supply loaded at a moderate fraction of its capacity runs cooler, quieter and usually more efficiently, which is a real quality-of-life difference.
So your 850 is not wrong. Where the cheap-and-large instinct fails is the next answer.