perhaps you just need to recalibrate your scales :o
I would suggest you overdose the filler say 20 grams the porta-filler will then be hard to put in the machine.
them remove or add coffee until you can feel the coffee touch the shower screen. at the appreciate position of the handle.
then weigh the amount of coffee.
but isnt dosing a volumetric thing not a gravimetric thing?
Throw the evil nasty scales away and just use the force Master Grobins.
Beware the dark side (that technology is a substitute for observation and brains)
unless you are keen on ristretto (that dark side can be an acquired but pleasant taste).
after all $7.00 for precision scales at the sensitivity needed ??? ;D
I would be testing the reproducibility of the scales very carefully.
if you really want to save beans use a single shot :o