Distance and speed are two variables. Changing both together is the single most common way beginners waste an afternoon and end up unable to say what fixed anything.
Phase 4 — Motionabout 30 minper filament
How to do it
Dry the filament FIRST. On PETG especially, stringing is moisture about eight times out of ten, and tuning retraction on a wet spool tunes around a problem you could have removed.
Find the setting: OrcaSlicer, FlashStudio, PrusaSlicer and SuperSlicer all keep it under Filament settings → Retraction, or Printer settings → Extruder. Cura calls it Retraction Distance under Travel.
Know the range you are working in before you start. Direct drive: roughly 0.4–1.5 mm, and 0.8 mm is a fair opening guess. Bowden: roughly 3–7 mm, because the tube itself absorbs movement. If your current value is far outside that, fix it before testing.
Easiest route: use your slicer's own retraction test if it has one, which varies the distance for you and labels each section.
By hand instead: print the two-tower model below at one distance, note the result, change ONLY the distance, print again. Three or four runs at 0.2 mm steps for direct drive, 1 mm steps for Bowden, is enough to find the knee.
Hold retraction speed fixed for the whole test. Two variables at once and you will not know which one helped.
Choose the SHORTEST distance with no strings. Going further does not improve anything and starts grinding a flat onto the filament.
How to read the result
The shortest distance that eliminates strings between the towers. More is not better — excess retraction grinds filament and causes jams.
Your slicer may do this for you
OrcaSlicer has a retraction test under Calibration.