@Ash_Crow @whknott @IHasWisdom I grew up with the imperial system (inches, foot/pounds and pounds) and had to change to the metric system. I still know that the palm of my hand is 4 inches (10cm), my hand outspread is 8 inches (20 cm) and my thumb is an inch.
But lets face it, 3 mm is far easier to visualise than 1/8 inch
It's merely a matter of utility. Whatever suits the requirement is the measure to be used and the result can be converted either way if necessary