Transmission with no dipstick and no oil level plug
CITROËN C2 Hatchback (JM)

Lost some transmission fluid whilst changing a spring, probably about a cup full.

After checking the transmission there is a fill cap and drain plug, triple checked all round and there is no dipstick and no oil level plug.  Have checked the Citreon C2 engineering manuals that show the transmission with a oil level plug but the version on my car is different and no part number on it.  Checked in engineering manuals that say the tranmission takes 2 liter of oil, but have read that if you drain the oil from the plug about half the oil remains in the gearbox.  So would that be 1 or 2 liters on a oil change; I have read a flush removes all the oil.  Is there another way to check the correct oil level.    Thanks in advance for any help.