Nice grab Steve. This subject seems to come up often when using tetrodes in Class C. Simplistic as it may sound, the venerable ART-13 modulation transformer used a tertiary winding to modulate the screen. Some have advocated the use of an audio choke in an effort to self-modulate the screen. This was pointed out to me when I was contemplating on using a 4CX250B(s) in Class C. Woody, N4MQ, found by adding a cap between the screen and the modulated B+ of his Johnson 500, he could increase his % of modulation.
https://www.youtube.com/watch?app=desktop&v=yXZBU3OPZdc This was a topic of discussion between me and Dave, W2VW on 160M one night and as he pointed out, he would like to have seen what it looks like on the aforementioned trapazoid test.