Trouble with scaling

I'm very new to LibreCAD which will be obvious I'm sure. I've read all I can find on scaling but I still can't make sense of it.
I have a part I need to make a copy of, with a modification. It's a flat plate with a couple holes and a slot. I need to add a couple more holes that are further out from the 'center' of the plate, beyond the current edge of that plate which will require enlarging the plate once I have the additional holes placed.
I've traced the part with paper and pencil, scanned it, imported it into LibreCAD and traced it there. When I show the properties of what should be a 0.4" circle, it shows as having a radius of 112 what I think are millimeters (properties seems to show millimeters regardless of how units are set in preferences), which is almost 9".
I set the scale set to 1, which implies a 1:1 scale before I began, yet tracing a 1:1 image resulted in this aberration.
The problem is that I need the drawing to be at a 1:1 scale. I have the polar coordinates of where the additional holes need to be placed. While I can get the angle correct regardless of scale, I can't get the distance from the center of an existing hole correct without having the drawing at a 1:1 scale.

How can I set the scale of the drawing to match the actual size of the part? I need to resize the entire entity, not the printing of it.

When importing an image you have the opportunity to set either a factor or a dpi value in the Tool Options bar above the drawing area. As you have scanned the image yourself you will know the dpi's.
The imported image then should have the right size as on paper.

Alternatively you can scale an inserted image afterwards with Tools - Modify - Scale. Measure a distance with the measuring tool on the screen and on the real thing, then calculate the necessary scaling factor. In LibreCAD 2.1.3 the image will seem to be disappeared afterwards, View - Auto Zoom will bring it back. This is a bug solved in 2.2.0 rc.

Thanks dellus. Yes I scratched my head over the disappearing image for a while until I stumbled on the zoom in, zoom out solution that brought it back. Funny thing is while it was 'invisible' I couldn't see the traced lines, but I could see smudges my fingers made while tracing.
I was just now looking at the circle properties and wondered if maybe it was showing pixels instead of inches or mm, but DPI solves the mystery. Doing the math with the radius property in DPI tells me I'm very close to 1:1. So a circle that should have a radius of 120 dots has a radius of 112 dots. Now I just need to figure out how to scale that and I should be set. Thanks again.