Extend distance not correct from Extend Surface

Hi all.
I dont know why the extend distance not correct when i rotate geometry, If i have 0° the extend distance is correct. But when I change to 35° is not correct. I don’t know if anybody can tell me what is wrong. Thanks in advance.
Test0508.gh (35.9 KB)

Suface extension relies on UV parameters so I wouldn’t give much confidence to it for precise offsetting. You may need to rely on Offset on Srf or in your case a simple Offset Curve since it’s planar.