What the value does
Pressure advance makes up for the filament that is squeezed in the hot end while the extrusion speed changes. Marlin describes its version of the value as the millimeters of filament compression needed for each 1 mm/s of extrusion speed. Prusa’s rule of thumb for tuning it: blobs on sharp corners mean the value is too low, and missing filament means it is too high.
Reading a tower
A pressure advance tower raises the value a fixed amount for every millimeter of height. Look at the corners and the seams, find the height where they look best, and measure it from the bed with calipers. Klipper writes the value as the start plus the measured height times the factor. Its own example, 0 + 12.90 × 0.020, gives 0.258.
OrcaSlicer’s tower works the same way with its start and step: a start of 0 and a step of 0.002 read at 8 mm give 0.016. Its line test needs no arithmetic, because each line is printed with its value and you choose the most even one.
Where the value goes
In Klipper, set it with SET_PRESSURE_ADVANCE ADVANCE= and the number. In OrcaSlicer, it goes in the filament’s settings, so each filament keeps its own. Marlin takes its K factor with M900 K in the start G-code, after its own test.
Prusa’s table gives different values for each material and for 0.4 and 0.6 mm nozzles, so a new filament or a new nozzle is a reason to print the test again.