Hi Timbor,
They are cheaper than buying washers! I have a pot on my desk and every few days I just take the bronze coins out of my pockets, I never have to worry about having basing material as so much stuff is sold for 99p I've nearly always got a pile of odd pennies.
I reckon leaving an extra 0.9mm would be a good fit, gives them a bit of wriggle room.
Rich