I have written above that I don’t understand why you are trying...

I have written above that I don’t understand why you are trying to calculate things that way.
There is no need to get kilograms of concrete involved in these calculations. Mass of a cubic metre of concrete even at the same strength standard can vary quite a bit depending on actual ingredients used.

More complex formulas give more chance for an error to creep in.

Concrete is produced and sold by m3 and all related information is based on this unit. We dose 1 kg of HRM (which is so fine that it doesn’t influence the volume of concrete) to reduce the CO2 emissions from manufacturing of all the components of that concrete and actual mixing process by 8%.
This drops the CO2 emissions by about 20kg per m3.

This said, if for the same inputs you get the same results it may be right.