sounds like you want the curves of the room perimeter with segments at the doors removed… not just deducted (which I took to mean subtracted, as in you were looking for a total length) – in that case, one approach is;

- find the perimeter of the room
- find the doors that belong to the room
- find a common point on the door (perhaps it’s centroid) and project it down to the same plane as the curves (not technically necessary, but easier)
- find the vector of the curve that hosted the door and draw a new line the width of the door and parallel to the vector based around the point you found for the door (if you picked the centroid, you’ll need to draw/offset the line in one direction, and then back the other way)
- take the end points of this line and project them back to the host line
- use these new points to split the host line and remove the segment

phew! i did this recently to create floor finishes going so far as to have the flooring push into the door opening so that the break between floors occurred midway under the door leaf - quite the task