In the above video, it mentions that you need to only pay attention to the sign of the strain value, not the direction of the coordinate system. For example, no need to be concerned, if say, the x-direction for one of your gauge points in the opposite direction of the other two gauges in the rosette. This is because the strain transformation is invariant when you rotate a coordinate direction by 180 deg. The transformation formula (see link below) has cos(2*theta) and sin(2*theta). When theta = 180deg, cos(2*theta) and sin(2*theta) will be the same as when theta = 0.
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