there are a lot of variables in chemical based photography.
folks who practice photography need to create a system/standard
that works for them, and that they have no trouble modifying to suit their needs.
In my opinion the only thing of importance is to develop a system that works for us as individuals. There are so many variables in how we photograph, stretching from camera and shutter tolerances (especially as they age), material choices, technique with metering and the light meter itself, not to mention how we print and final output requirements.
My Gossen Luna Pro does very well at box speed for me. My in camera meter on the Canon EOS 3 likes a lower than box speed EI for the most part (about 2/3 stop), the Pentax KX requires me to set the film speed at half box speed, etc.
The trick to becoming a good craftsman (craftsperson?) is to know our materials well enough to know what to do in all kinds of situations. Nothing else really matters, regardless of what the box says.
Well, perhaps you can view box speed as a kind of starting point?
Then different films behave differently. Some like both being pushed and pulled, some like one but not the other and some likes neither.
Colour films are different from B&W too, usually requiring you to stay pretty close to box speed.
The films and developers are not the wildcards, we are, all us nuts behind the cameras and agitating our films.
The materials we use are remarkably reliable and consistent as long as we do our part.
Our vision for the end result and our preferences in how we like to do things are what drives us to deviate from "normal". These decisions are regularly poorly informed and done for irrational reasons.
As to color forcing us to stay close to box speed, that is at best an urban legend. Kodak's old Portra brochures claimed consistent color from one stop under to two stops over. Disposable cameras are more proof of that.
I went through the bulk of Fomapan 400 recently. Most of it in Rodinal.
Have hard time to believe it needs twice time for stand developing and twice time for fixing. But true.
Best results were at ISO 200 for me.
Same with Kentmere 400.
Back when film speed was determined by the ASA (American Standards Association) a specific developer was specified. The formula for which was part of the ASA standard as it was not a commercial developer. Now with ISO standard the manufacturer is allowed to pick the developer that they wish to use. Different manufacturers can use different developers. I personally have never found this idea to be particularly attractive. As the old saying goes "The nice thing about standards is that there are so many of them."
Certain developers like D-76 and Rodinal (with other analogs like R09) do not produce full rated speed. So its is not surprising that Fomapan 400 only yields an EI of 250 in Rodinal. Manufacturers design their films and developers together. My advice would then be to look into one of Foma's recommended developers like Hydrofen. I published the formula and developing times on APUG some time ago.
So you are not far away,
In 1+100 60 mins at 30C Rodinal &
In Microphen 9 mins at 20C I use 320
I use a Weston incident or zone1 reflective and 200 might be better, I hate empty shadows and open up a stop for non Caucasians.
It fixes quite quickly for me unlike the tabulars which take aeons.
This comment is appropriate here then!
Traditionally, EK ran a release process for all B&W films to verify the speed and curve shape of all films before going out into the trade. The developer used was D-76. They may have changed, but when I was "there" that is what was used. And, the speed ratings were given for an approximate contrast of 0.6 as I have noted before.
Thus, if a Kodak film is developed to a contrast of about 0.6 - 0.8 (professional to consumer range) the speed should be spot on for the box rated ISO speed.