Grading Multiple Countries For An Inventory List

Recently I discovered a wonderful site for trading coins.
It allows for the creation of an inventory list.
Five different grades are allowed:

VG F VF XF UNC

The site is bi-lingual, and uses the French terms:

B TB TTB SUP BU

which I conclude to be:

bonne - très bonne - très, très bonne - supérieure - BU

Any coin from bent to well-worn I list as VG.
I use XF if a coin still has luster present.
It is the the F and VF grades that are giving me a problem.
Any ideas would be of help.

At present, I am using "F" if the coin has sharp rims and
"VF" if the central design has three-dimensionality.

Thank you,
Matthew