how to fillet this one?

 From:  wimverbe
7911.1 
a very simple path, but I am not able to fillet this one (except by adding 2 points and deleting the corner point, but I don't get the accuracy I am after).
I want a fillet of 0.5 cm here, which is small enough to fit between the existing points.

any ideas?

tnx
wim
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