The rationale goes that texture will keep the flow attached for longer, so if the smooth part is really a third of the circumference and as such does not go past the widest part of the leg, then it should sit in front. Depending on leg shape and separation point, they might work equally well both ways though.

The textured part goes in the front. The idea is to trip the boundary layer turbulent prior to the point of laminar detachment. As the PP mentioned, that is about 1/3 of the way back (at 0deg. yaw), but will vary based upon your calf shape and the yaw angle.



In this view you can see the calf muscle is on the smooth part.

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