A ruler doesn't tell you anything about measurements?
I suggest it will help in the discussion to look at CCP 192 to structure the argument. In particular, one should examine both Clause 1 and Clause 2:
Article 192
Evaluation of evidence
1. The judge shall evaluate evidence specifying the results reached and the criteria adopted in the grounds of the judgment.
2. The existence of a fact cannot be inferred from circumstantial evidence unless such evidence is serious, precise and consistent.
In order to evaluate evidence, one must specify criteria for the evaluation. That means, for example, a theory of relationships and a procedure for taking measurements important for showing relationships, if any exist.
Furthermore, based on ECtHR rulings, an arbitrary trial violates Article 6 of the Convention; therefore, one should not use arbitrary reasoning to form criteria (my interpretation).
For the footprint/track on the mat, the prosecution and defense had different theories and procedures for evaluation. One must compare them, and be confident that the criteria chosen is valid and not arbitrary.
Then one can proceed with the second clause. It is possible that one may conclude, even after carefully and rigorously following the criteria, that the result is not sufficiently specific or precise. That is, a specific person cannot be identified with certainty. Then one has to at least momentarily set aside this circumstantial evidence and continue with the other, more probative elements of evidence.
ETA: Of course, the result of the footprint/track analysis may be useful in excluding certain persons.