Can you do that in your head? I can't.
No, me neither. The calculation is the purpose of the calculator.
What I am attempting to add to the conversation is that the same calculator and same result can be used for fstops, for inverse square law distance, and for guide numbers. But these all work relating to the square root of 2 (1.414), so third stops sort of blurs it.
If you wanted to know how light fell off at 80 feet vs 40 feet, enter f/80 and f/40 there (as if it were distance), and the answer is the correct 2.00 stops.
Or 4.4 feet and 2.2 feet, same 2.00 stop difference.
Double distance is of course 2 stops, inverse square law.
Shutter speed is linear, not the same thing.
And a third of a stop is generally regarded as the smallest increment where you can detect a measurable difference in exposure, even using a densitomer. Two decimal points is superfluous. Most camera shutters and apertures are not any more accurate that that.
Tell that to Sekonic.
Their specifications for our light meters is to detect and measure to tenth stops. We have a tenth stop mode, which the specs claim to honor.
Can we set it on our camera? No (or rather, we have to round it off). But we can set our multiple flashes to that precision, honing in on their ratio.
The biggest advantage of tenth stops is: How many stops lighting ratio between metered f/6.3 and f/10? Not that easy, and the purpose of the calculator.
If working in tenth stops, it would read as f/5.6 plus 3/10 stop vs. f/8 plus 6/10 stop. So we DO know this in our head, immediately. 1.3 stops.
Here is a tenth stop table using three significant digits (which are necessary to meaningfully create the table):
Photographic Tables, Aperture f/stop, Shutter Speed, ISO and EV