Probably the thing to do if you one wanted to pursue this kind of Gedankenexperiment would be to create an Excel spreadsheet. Wherein you could plug in different heights for the rod butt end of the rod, accounting for different heights of the angler or if kneeling down, different lengths of rod and line, and different angles of the rod. For angles it might be at with the fly in the water. Say 30˚ to 75˚. Make some assumptions, of your choice, about where you would want to call it a red zone, that is points where you could land the fly or pull it to, but would be points where the cast would likely spook the fish due to to much line hitting the water, or an angle with to much line sag, or where hook sets would be poor. (while a spreadsheet like that can make quick calculations for changing criteria, once it is set up, I prefer old style slower way of drawing it out, it just always seemed to me that process helps me think about it better. Kind of the thing where research shows people remember things longer when written out by hand than when typed into a word document) However, while I find doing this sort of thing interesting, at the moment I have other interest taking priority for my time.
You’d probably discover some surprises about the size of the area on the water the water that you could cover with different combinations of rod & line lengths and rod angle. And end up making some changes in your set up.
otoh, you might find from the theoretical idealized water coverage that would be the best set up for what you want to do is a 4.5m rod with a 6m line, only to find out in practice it doesn’t work for you because you lack the skill to cast a 6m line anywhere near to full extension of the line.
Anyway, speaking of line stretch. And to add to the complexity of line choices. The other day I posted a link, in the horse hair line thread, a DT TiF S 1 Episode 1 video. Where Go Ishi was interviewing Yamada-san from Aikyamagou. A little later in the interview after the bit where he was making a horse hair line.
Yamada-san tells about teaching beginners how to tenkara fish. And he describes how they have two problems with hook setting. 1)they are to aggressive, swinging their whole arm high, instead of doing a faster wrist movement, and 2) surprise hook sets, aka poor timing. His recommend cure is to tell the students to use a thicker tippet.
Now just how and why would using a thicker tippet offset these beginner hook set problems? I have to admit I can’t really tell much difference between using 5x or 4x tippet. Though I can sense a difference during casting of a 1m ~ 1.5m 6x or 4x tippet. Casting a 4x tippet to full extension is easier than 6x. But not a difference in the hook set. What is accomplished by using a thicker tippet is something I’ve been pondering, but not yet any idea why Yamada-san recommends it.
See it here, it runs about 90 seconds before moving on to a different topic.