Dear Petar,
Thank you for the question and I am very sorry about the experience!
Unfortunately, it is not possible to edit the reference line thickness.
However, if we set the Snap Guides on and position the mouse next to the desired corner, a blue circle will appear to assist accurate selection and replacement. Please find the Help Center article below:
https://helpcenter.graphisoft.com/user-guide/127969/
Please let me know if it helps with the issue!
Best regards,
Minh
Minh Nguyen
Technical Support Engineer
GRAPHISOFT