if you just want to record something for set amount of time, you can set frame count on the recorder to be [seconds]*[fps], so if you want to have a 4 second physrecording with smh's frames per second being 30, set amount of frames to 4*30=120 frames.
make sure smh's playhead is at the part of the timeline you want to start recording from, and phys recorder will also overwrite existing keyframes, if it comes across any