How to create geometry in a layout viewport that is the same scale as the model

Sometimes I need to draw lines of a certain length while in layout. For example, often I need to draw a dashed line to show a hidden feature, and it would be a lot easier if I could do this after I have inserted my model into layout. Unfortunately, my line lengths are in papers space and I want them to be in model space, or at least be the same scale as the model I am working on. Any suggestions?

This is my way:
draw the dashed line and let the length snap, open the model by cklicking the right mouse button (don’t doubleclick the viewport) and delete the line in the modell. Save the modell to come back to LayOut in same scale and position. After that never hit Ortho, it will messup.

Thanks Barbara for replying, I think that will work. I also came up with a workaround that was very surprising. After the model has been inserted, I go back to the model and trace over what I want to be dashed, then simply copy and paste the outline into layout, and then scale it and apply the line style I want. I’m not very good at putting these keyboard and mouse sequences into words, I hope it make sense. Thanks again.