I propose Rexel Workflow when it comes to setting texel density while unwrapping. This workflow is for static images and is camera-dependent. Rexel is the final rendered pixel.
Let’s say we have a scene with a vehicle model: an airplane with detailed objects and a few cameras, some distant for an overall view, but some for close-ups of parts of the fuselage. Now we want to find an optimal ‘Goldilocks zone’ for texel density. We can do it by hand using the PureRef reference grid floating above a window of unwrapping software. But it would be interesting to have such UV unwrapping software that can calculate and set Td for us - based on the rexel density. More texels per rexel = higher rexel density [Rd]. Of course, too much Rd and we end up with too many UDIMs and VRAM constraints. So there must exist some Goldilocks zone. This zone might have Rd of values between 1.5 and 2.0 - when 2 pixels of texture constitute 1 pixel of render.
What do you think?

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However ..
You must account for FOV(I recommend vertical fov for PC/console). screen resolution and view distance are not enough to give you accurate numbers
It can also be boiled down to a simple equation that gives you an ideal texel density for a given distance (with fixed FOV and screen resolution) - no need for any fancy images etc.
I'm tired and slightly drunk so I won't attempt to write it out now but it's basic trigonometry
It's worth mentioning that your GPU knows about all this - it's how mip selection works.