u/aanth79

This is all personal opinion and some attempt to retrospectively apply recent data to existing canon (obviously if you accept that Doctor Who has "canon" and is not exclusively wibbly wobbly timey wimey).
I don't think it's an equation with a solution, or it's not directly translatable to current earth mathematics.

If we take δ³Σx² (d-cubed-Sigma-x-squared) as a name or designation then it could have this intepretation.
The first part, δ³, could mean a small change related to 3. At that time, they could have been the third iteration of something, a third child, a third version, a third reincarnation. Σx² could be some kind of reference to a total or an aggegation of a whole group, species, family, test subjects, a second familial generation or test level. In the simplest interpretation it could mean the 3rd regeneration of Σx². θΣ (Theta Sigma) could be a more informal given name replacing the δ³ and Σ is like a "family" name.

After considering all that, in my head, The Doctor's designation relates to the third regeneration of that lifeform and the latter part in someway related to the whole Timeless Child/Shobogans/TimeLords origin arc. Which would also mean that the designation changes with each regeneration, though I could never say for certain which regeneration we are on.