FWIW for my last brew I mashed 50% malted wheat and 50% 2-row. I only hit 57% efficiency (normally I am around 72%). Iodine test confirmed complete conversion, so my thoughts (after consultation with the good folks here at HBT) are that I had some channelization during lautering and/or a poor crush on the wheat malt. So, next time I do a 50/50 recipe like this I am going to use some rice hulls and have my LHBS crush the wheat twice (or adjust the mill). Just something to consider.