I'll be honest with you : doing 3D in Autocad with a too light knowledge is not good...
To deal with units in target and sources means that you 100% manage blocks outside your drawing, that you know how to purge, etc.
Would it be a block or not, I would WBLOCK this door and open it as a single DWG, then UNITS command in it and see if coorect.
Back in the house, where the door has been purged (if it was already a block), I bring it back with INSERT and Browse....
Again, when sources and target are well set, no worries...
Another way would be to SCALE the door using the REFerence option with no question of blocks. It will ask you a base point, you'll choose then the Reference option and will specify the width (opening) of the door with 2 clicks (for exemple) and the final length, that you may know (870mm)
If you know the scale from the object and your calculations, it is even easier (looks like your door needs a 0.1 factor)
PS : in recent Windows, you can find the little soft named "capture" to take photos of your screen.