Mr Kirk
You are just making all of this TFC! the three times rule works in my Robin DR400 just as well as it works in a B738 or an A320.
It is not perfect, it will not give you an exact TOD to get you to an exact point in space but it will get you into the ball park without using too much brain power, I have no doubt that by using a complicated formula you could much more accurately fly into a hill that you had forgotten about due to the accurate and time consuming calculation.
Remember single pilot IF is about keeping the "big picture" rather than micro managing.