Quote: "FWIW: I often replace some of the DME (75% fermentable) with sugar (100% fermentable). I also use a higher attenuating yeast (like US-05, and unlike Windsor)."
I don't know why the difference, but I generally get attenuation in the mid 70's to mid 80's when using LME and DME - depends on the specific style and yeast. Yeast is mostly US-05, Nottingham, S-189, BE-256, and Munich Classic Wheat Yeast. So my thought is try it without substituting sugar first to see if it is actually needed, given your particular process. Obviously, ymmv.