How would one begin to calculate the availability of nucleotides to an enzyme ? What units would be used ?

I imagine an answer like nucleotides per second per enzyme, but I'm also imagining an answer that is not the same as processivity.

An experiment that selectively highlights all entries of correct and incorrect nucleotides into the binding site of the polymerase would yield such an answer.

I'd like to know, specifically, if there is a theoretical approach to this question.

How would one calculate the availability of nucleotides to an enzyme like a polymerase ?

I imagine an answer in units like nucleotides per second per enzyme, but I'm also imagining an answer that is not a simple rate of reaction. k_on calculations and k_off do not offer mechanistic explanation.

I'd like to know, specifically, if there is a theoretical approach to this question: how does a polymerase reject an incorrect nucleotide?