If you determine the middle exposure then automatically have +/- 2 stops then you should cover the DR of the sensor plus 4 stops
Technically that's correct, except what exactly does middle exposure even mean? A small very bright patch can throw it to some very unexpected numbers if we go by 1/2 of the brightest exposure.
You may cover it all but perhaps it will be quite suboptimal (+1EV) in key areas.
+4 stops is actually not all that many by the way.
I will usually automatically choose 5 shots at 2 stops interval.
By all means that is probably the only way to get a safe exposure the first time with the antiquated "auto" bracketing. You will likely throw away 2 shots immediately.
Also changing from 3 to 5 and back to 3 on a regular basis is a right PITA.
estimate by using live view and the histogram
Guesstimate. I stress that again. You are looking at dodgy 8 bit over-processed JPEG histogram that is a long way away from your RAW files. You are likely under exposing it by 2/3-1 stop based on that (tip set profile to Faithful and AdobeRGB to bring it ever so slightly closer to truth. Video CLOG2 / REC2020 is what you ideally want). When it's really dark histogram starts to become completely wrong in live view (on Canon).
I also notice Canon histogram doesn't clearly display blow highlights in very small areas that you really might want to preserve. The whole thing is a terrible guesstimate because you start with unreliable data.
Besides on Canon you have to press INFO 4 times every time to get histogram and back to level indicator. You can't have both at the same time. That's super inconvenient. It could only get worse if I have to prepare a written shooting plan at home before setting off.
To suggest all other methods are pure guess work is insulting
See above and try again. That was aimed at me? I couldn't care less in fact.
Also I am happy just to use two of the three exposures if that is all that is needed so see no problem there.
If you are happy with that then fair play. I am no longer happy with at all. Throwing 1/3 or 2/3 is very wasteful and inefficient process, all because we can't get true histogram.
P.S. You may find as a first guesstimate the Canon's HDR MODE from the menus does a pretty reasonable job estimating the bracket range up to +/-3EV; throw away the combined JPEG and keep the RAWs, then expand from their values if necessary. You are still limited to 3EV in 3 shot mode, so may as well just do the single missing frame.
None is this 1980s b******t is necessary with Magic Lantern bracketing implementation. What on earth is stopping camera companies from enabling the obvious?!