Your translation will be "Principle of Sufficient Reason" followed by one of the concepts contained in the links. I would say that obtaining the calque "absolute determination" from "completa determinación", we could consider:

"absolute space [and time]" or "invariance" and the confidence that it provides...

as a very good possibiliy.

Research links:
https://plato.stanford.edu/entries/leibniz-physics/#SysConPr...
https://en.wikipedia.org/wiki/Philosophy_of_space_and_time#L...
https://iep.utm.edu/leib-met/#:~:text=While Leibniz's p...

The other possibilities that I found for Leibniz to do with "completa" were:

"Invariance"
Invariance, or symmetry, applies to objects, i.e. the symmetry group of a space-time theory designates what features of objects are invariant, or absolute, and which are dynamical, or variable.
https://en.wikipedia.org/wiki/Philosophy_of_space_and_time

Absolutism (however, this is translated on Wikpedia as "Absolutismo") which is this idea of "absolute space and time".
Absolutism and relationalism
Leibniz and Newton
The great debate between defining notions of space and time as real objects themselves (absolute), or mere orderings upon actual objects (relational), began between physicists Isaac Newton (via his spokesman, Samuel Clarke) and Gottfried Leibniz in the papers of the Leibniz–Clarke correspondence.

https://es.wikipedia.org/wiki/Filosofía_del_espacio_y_el_tie...

and the "Complete Individual Concept" (but it seems a stretch given your source text).

Perhaps there is more context elsewhere in your source text that could make the choice clearer.

"absolute certainty" refiere a algo absolutamente seguro, sin lugar a dudas, así como también hace referencia a un punto de observación que pueda tomarse de un determinado hecho.
The principle of sufficient reason provides absolute certainty and trust on the matter of the existence of a sufficient explanation for everything that exists.