Nozzle opening usually is linear with thrust. ...or used to be, but now days with higher internal pressure we see more oumpf through smaller outlets. K70 has exactly the same nozzle as K45, I compared at Jetpower. If the K70 was the real deal I do not know though. I know some engines do not like back pressure such as Frank, while others do fine. My friends flies the Hawk 240 on a 75mm pipe. (Might have been the 190R too, but still 200N+ on 75mm tube.)

T50 is 33mm on my CAD model, engine installed at the moment so can not double check. But I know I had engine plus calipers when doing the model.

My bottom line is that people should test more. Even though grumania and Tamjet make awesome tubes I think they are not testing enough and selling shelf parts. I run a 46mm tube for 5.4kg thrust with video to proove I gain 200grams with the tube while Grumania suggests the 65mm pipe as it's listed 40-80N.