Value C is a multiple multiple reference. I do wonder why. Will a new added value for C also be reused by other users so they can just select that value again for their calculation? Is a multiple (from the C side) to one (the Secondmilestone) not simpeler. This way you could create a listview in the page where the user could create new values for C and add it to the list.
For the calculation you could then retrieve in a microflow the list attached to the Secondmilestone object iterate over those to do the calculation.
If multiple multiple is needed you need to add a reference grid to your page and create a select page where the user can add the C values or create new ones. Then the microflow is almost the same and retrieve the C values attached to the Secondmilestone object.
Hope this helps a bit.
Regards,
Ronald