How to calibrate the retraction ?

This is one of the most important parameter for print quality and for reliability.

Temperature

Retraction is about first temperature, second temperature, third temperature.
So you know how important it is.
For instance, typical PLA parts can be printed flawlessly at 200C, but miniatures are printed at 175C to achieve good quality.

Retraction length

If the length is too short, the retraction will not be effective.
If it is too long, you have the risk to clog your hotend and ruin your print.
It is needed to be long enough and be very reliable, especially when you have successions of retractions.

For Bowden setups and PTFE Lined Short Blocks, I found that around 6 mm (+- 1mm) is a good starting point for many PLA, PETG and ABS filaments.
For Full Metal Short Blocks with a Bowden setup, you can start between 2 and 3 mm.
For Direct Drive, it depends on the HeatThroat and the amount of play, you can start between 0.5 mm to 3 mm.

Retraction speed

It depends on your material, some materials are very viscous and they need a slow retraction, some material like a bit faster. The firmware settings can have also an impact regarding acceleration.

For some sticky material like PETG, I like to use an asymetric retraction with 40 mm/s to retract and 20 mm/s to reprime. It improves the reliability on retraction intensive parts.

Move speed

If you let a nozzle at print temperature it will ooze, so a good move speed helps to prevent oozing (but to not over do it to keep it reliable and prevent shifted layers).

Avoid retractions

You should always use the sequential island partitionning to reduce the number of retractions, and optionally the avoid crossing perimeter options in the Travel moves.

Wipe Before/After Retractions

For Bowden setups it can help to release the pressure inside the nozzle and obtain a cleaner part.

Testing procedure

Quick Testing: the goal is to set the print speed as usual (disable speed cooling parameters), use basic retraction settings. First, take the time to tune the temperature, then retraction length.
Thin Testing: once the quick test is finished, perform a first iteration of the thin retraction test (reenable speed cooling parameters) and start to tune further the temperature, the retraction length, then retraction speed.
Stress Testing: once the thin test is finished, I like to use miniatures about 28mm height to stress test the retraction on real cases. It uses little filament, it is difficult to print: it is very tough on retraction settings for accuracy, quality and reliability. If you can print cleanly miniatures, you can print anything.

Hardware to check

Worn out nozzle or leakage
PTFE Tube wornout inside the hotend (if not Full Metal hotend)
Bowden PTFE Tube wornout outside the hotend
PTFE Couplers or any other source of filament play in the filament path for Bowden
Fan cooling the Heater Block/Nozzle, it can affects temperature reading a lot
Heater Block insulation, it can be useful to reduce radiated heat on the printed part