You can calibrate it accurately by measuring it. For example in Cura, you load in a solid cube, set line width to 0.5mm, wall line count to 1, top layers 0, bottom layers 1 and infill 0. That prints a hollow cube without a top and single wall count. Then just measure it with calipers, your target is 0.48. No idea why it's not 0.5 like the line width is, but that is what all the guides say
.
The formula for new flow rate is (0.48 / measured thickness) * current flow rate.
It can get quite low, for example I have to set my flow rate to ~80%. In practice I have noticed that the calculated theoretical flow rate has always been a little bit too low, I can see that the top of my prints have had small gaps and I've needed to adjust it up a bit. Theoretical numbers are a good starting point, but the real adjustment should be done by observing the actual prints.