In a isometric perspective, done correctly, we should apply a reduction in every edge. That reduction is the same for the 3 edges, and it's 0.816, that you can calculate why, but it's that number. In LayOut, simply prepare a custom scale with that number (1.0 mm = 0.8166 mm). It exist differences between an isometric sketch (no reduction) and a isometric perspective (0.816 reduction on every edge). Now you can print and mesure correctly your isometric drawings.

I hope it helps someone.