Hi,
From the Günther Rosipal J.G. 54 loss list, which can be found at
http://www.jg54greenhearts.com/JG54list.htm:
On 18 December 1944
Uffz. Dieter Schmitt of 13./J.G. 54 was posted missing after a combat south of Cologne in an FW 190 A-8 marked 'White 11 + '.
Also to be found on that page is a J.G. 54 victory list by my friend Nick Hector. However, I can find no aerial victory claims by Dieter Schmitt.
Hope that is of interest.
Cheers,
Andrew A.