I think I have a scaling question

Hello All,
Using Rhino 4.
I have jpg’d a scan of a blueprint into the background using picture frame.
I have set up multiple layers for the setup and part lines that represents the poorman’s digitizing I am doing.
All lines are planer at this time but I have some 3D construction to do, (turning flanges and surfacing for metal thickness) so I feel now is a good time to scale the drawing correctly. I suspect it is pretty close in that when I draw a called out circle dimension it matches reasonably well taking into consideration paper stretch and scan inconsistencies but accuracy is everything and I would like to make sure it is correct.
I assume there is something not right because I drew one side and mirrored it over using a print centerline and it missed matching up to the other side by about .010".
My questions are:
How do I go about using a correction factor?
Is it going to be global to the whole file or layer specific?
Should I have done it earlier in the process?
Thanks for your time, Rick

Hi Rick- I’d look for a known line that is meant to be horizontal or vertical in the plans- known in that you know its exact length and its exact orientation, preferably horizontal or vertical. The longer the better.
Then, I’d use Orient, with Scale=3d,
For the first and second points, I’d zoom in an pick as exactly as possible on one end of the line, then the other.
For the third pick, go back to the first point, or hit Enter to automatically use the first point.
Now, if the line is horizontal or vertical, hold Shift for Ortho and type in the exact length of the line as a distance constraint, (with Enter). Click to set the horizontal or vertical orientation.

Any luck with that?