Flow rate, extrusion multiplier and flow percentage are three names for the same thing: a scalar on how much filament the extruder pushes. The reliable way to set it is to print a wall of known intended thickness and measure what you actually got.
The test print
Print a hollow cube — 20 or 30 mm is plenty — with the number of wall lines set explicitly, no top or bottom layers, and no infill. Two wall lines at 0.4 mm should measure 0.8 mm.
Let it print at least 10 mm tall so there is somewhere to measure that is clear of the first layer, which is squashed and always reads thick.
Measure in four places, away from the corners, and average. Corners are thicker because the nozzle slows and the pressure has nowhere to go.
Why the correction is a simple ratio
If the wall should be 0.8 and measures 0.86, the extruder put down 7.5 percent too much material. Scaling flow down by the same ratio puts it right.
New flow equals old flow times expected divided by measured. That is the whole calculation, and it converges in one pass — a second iteration should land within a couple of microns.
It works because extrusion is very nearly linear in this range. Push 7 percent less plastic and the wall gets 7 percent thinner.
Steps per mm first, flow second
These two settings do overlapping jobs and that is why people go in circles. Extruder steps per millimetre describes the machine — the hobbed gear, the gear ratio, the motor. It is set once.
Flow describes this spool — its actual diameter, its material, its melt behaviour. It changes when the filament changes.
If you compensate for a machine error with flow, every new filament needs the machine re-checked. Calibrate steps by extruding a measured 100 mm of filament in air, then leave it alone and use flow for the rest.
When a big correction means something else
A correction of more than about 10 or 15 percent is a symptom, not a setting. Filament diameter tolerance accounts for a few percent at most; material differences a few more.
Look at extruder steps, at whether the filament is really 1.75 mm, at a partially clogged nozzle, and at whether the extruder is skipping under load.
Dialling flow down to 80 percent to compensate for a mechanical problem produces prints that are dimensionally acceptable and structurally poor, because the walls are thin and the layers are not bonding properly.
What flow does not fix
Flow is a single multiplier on all extrusion. It cannot fix a part that is the right wall thickness but the wrong outside dimension — that is a steps-per-mm or a shrinkage question, handled by the print scale calculator.
It also cannot fix elephant foot, stringing or corner bulge. Those are first layer squish, retraction and pressure advance respectively.
Set flow from a wall measurement, then leave it and tune the other things separately. Changing two variables at once is why calibration sometimes never converges.