Hi Michael - I guess I am not coming across very clear again.

I always start my projects as "No Unit System", so if I do need to be using a background image as a tracing template I then draw a box to the same dimensions as the image, so if the images is 800 x 600 the box is drawn to the same scale as the image. The box is then positioned as mid point or centre point to origin and the image is then loaded and placed using the snaps provided by the box geometry. Once placed the box becomes locked or hidden or just plane old deleted, but once drawing/tracing commences I am still using no units system and effectively speaking the scale becomes 1 pixel of the image = 1 no scale unit of MoI, hence me saying that I am using the pixel size as a unit size. Once all the drawing is done, you can scale to what unit of measurement you need or just plane change the unit of measurement to whatever you need, feet, inches, mm etc.

All of this works fine for me right up to the point of export when the OBJ files, no matter what the unit size is, are exported as huge mesh objects. Once the object gets imported into other rendering packages they come in as massive objects, but again this can be fixed in the export mesh options by using the scaling tool provided in the OBJ exporter. If on the other hand you are using rendering engine such as KeyShot that does not seem to have a world limit until any objects are imported in then you don't even need to worry about the scaling of the exported object

I hope I have made a little more sense this time with my rambling - All the best

Martin Spencer-Ford