Overboarding thickness - how much smaller will my room be?
10 mm &quot;truss&quot; + 13 mm plasterboard (taking the high end) = 23 mm
23 mm x 2 (both sides) = 46 mm
10000 mm wide corridor - 46 mm = 964 mm

Your estimate of 950mm of remaining space seems reasonable.
Other than checking your math, I'm not sure what else we can tell you or what your real question is.