Not sure your questions is clear. What do you mean by "WFB glider 100%......"
The white face trait is dominant so there is no way to tell if the WFB carries one or two genes for White Face.
A leucistic glider is by definition 100% in that it takes 2 of the recessive gene to produce a leu. If you meant 100% Leu Het - which would be a grey glider that had a leu parent so you know it carries one of the recessive genes, you would only get grey or white face gliders.
To get a leu glider from a pairing of a leu and a WFB - the WFB would have to also carry the leu gene.
All joeys from a leu parent are 100% Leu Het and might produce a leu in the NEXT GENERATION if the 100% leu het joey is paired with another 100% leu het.