Here is my simple good old trick. You have to know the distance (as long as possible) between two points in the scanned picture. Paste it into the plan somewhere, don’t pay any attention to resolutions, scale measures etc. Draw a line from point A to B. Zoom in to gain accuracy on both ends. Select both the picture and the line. Go to Edit>Resize..., tick “define graphically”, then <OK>, click on point A, then click on point B, leave cursor on point B, hit “r”, type in the known distance and hit <enter>. So now you have your plan picture in correct scale. Delete the ref line, place the picture in desired position and angle and you’re done it.
