If you have 20m of cable at 20W/m you need to add 400W. You are NOT adding W/m2 you are adding W/m3.
In the CFD model you have the lines. It's 20m long, and 1m deep with a thickness (wall bc Thermal tab) which gives you the volume. Divide the total heat input by that volume, to get W/m3 and put that value into the panel.