Actually, Gabor, I do understand this, thanks to your posts at GetDPI and elsewhere
I'm sorry, I did not recognize your tag from other forums.
I'm still not completely clear as to which is the very best ISO to shoot this back on
The ISO setting has to roles:
1. evaluation of the necessary exposure,
2. instruction of the raw processing.
You should figure out, which setting yields the most accurate metering. Try to achieve ETTR using the in-camera histogram. If the P25+ displays a raw histogram, that's great. Using a neutral WB (Uni-WB) is not an option, if your customers are standing behind your back and want to see the result on the LCD.
So, shoot the same scnery, same illumination with metering based on different ISOs, and verify the actual, i.e. raw exposure. You can use Rawnalyze, but you have to convert the raw file in DNG (Adobe's DNG converted does this very fast).
If you know, which ISO setting reflects the raw data state the closest, stick to that. Meter and shoot with that setting, as long as you can. If the light is not enough, the shutter would have to be too long, reduce the exposure with bias, and be aware, that you are now working with "ISO 400" or "ISO 800".
The ISO 800 image of Buy's bathroom with the P45+ was almost three stops lower exposed than the very right. When you process that in ACR, it becomes increased by 2 EV. That is quite noisy already (though the noise is nice). Two stops from the right edge is quite good.
If C1 treats your images as clipped, even though they are not, and you can't help with exposure/brightness correction (I don't know, how C1 is acting in such circumstances), then go with a lower ISO; that makes C1 believe, that higher pixel values are "acceptable".
Finally, pls post a raw file of whatever with whatever setting, except a black frame, only as technical reference for me. I am hunting for the error, which I noticed in the raw file of the P30+ posted above by 203.