The results are only inaccurate because I didn't average the end results (I tried to keep it simple for efflux). Plus I only used 2 steps in my last example. In my example before that I used 4 steps and you could see that the estimated time didn't change much by the fouth step.
When TG2 renders, it takes many, many steps (other 3D programs call them buckets, but TG2 does not render steps in any ordered fashion or shape, thus they are not truly buckets), which would, when averaged, provide a far better estimation.
Doing a low res render then a high res render, not only is redundant (waste of render time), but would only serve to provide a barely usable estimation. If I had an image 720 x 405 that rendered with a Detail of 0.1, and AA of 2 and it took 1 minute. How does that tell me how long a render with a Detail setting of 0.7 and AA of 4 will take? It could take 10 minutes, or 60 minutes...
Don't get me wrong, I am not trying to make a big deal out of this, just trying to lay to rest the claims that it is a difficult thing to do, when it is quite simple