So, we now have another abbreviation in the swimming jargon: SF :-) ?
Again, you didnt pick the right start point. The start point is not where the hand is still gliding forward, but the point where it starts pressing water back. Thats where the slipping starts....
:-) It's already in the list...
But then you'll "never" find a starting point, that can't hold for further discussion. In Terry's and in Shinji's example I started with the point where their hands hooked to move backward (related to their body) to the point where the same hand exited the water... same as you did with your straight-arm-example. So "my" SF is more or less the relationship from arm/hand-movement under water (related to a fixed point) to the movement of the head (related to fixed point). That's indirect the connection to my estimation realted to SPL. Every other points seems (to me) much more subjective, as you stated, how to include the amoutn of kick? With an extreme kick you might get a SF=0.
But, its fun that someone else is also interested in this stuff.
Hmm... not sure what could be taken from it for own swimming and coaching. SPL and SR seem to be the "harder" and more reproducable reference numbers.