What I did was set up a coordinate system with (0, 0) at the back bottom of the couch, then determine the equation of the yellow line in your picture: it passes through (0, 970) and (930, 600) so has equation y= ((600- 970))/370)x+ 970= -.398x+ 970. The found the equation of the blue line- it passes through (0, 0) and is perpendicular to the yellow line so has slope -(1/(-.398))= 2.51. Its equation is y= 2.51x. They intersect where -.398x+ 970= 2.51x or x= 333. Putting that into y= 2.51x, y= 837. The length of the blue line is mm.
If you take off the feet, the ends of the yellow line will be at (0, 930) and (930, 560) so the equation of the yellow line in this case will be y= ((560- 930)/930)x+ 930= -0.398x+ 930. The equation of the blue line will be y= 2.51x. The lines will intersect where 2.51x= -.398x+ 930 so 2.91x= 930 and x= 319 mm. y= (2.51)(319)= 802 mm. The length of the blue line is mm.