Class L bobbin tension meter jig.

Glue two 8*2mm magnets on the bottom of the print to have it attach to your scale. Print with the pin facing up.

Insert the bobbin case with the bobbin inside so that the hole/slot on top in aligned in the middle of the two pins/bars. Make sure you open the latch when inserting and then close it to lock it to the gauge.

Slowly and consistently pull the thread perpendicular to the scale (upwards). The average of your readings should be between 18g-20g tared. Make sure you use an accurate scale (500g-0.01g ones on most stores are decent enough). In theory, this should properly measure the thread tension.