As far as I know, when someone says 'a reduction is tight', it means that given that there is an adversary $A$ with advantage $\epsilon$ and running time $t$ and another adversary $B$ utilizing $A$ to solve a problem $P$, the advantage and running time of $B$ are apporximated to those of $A$.
But here is my question:
When do we say $\epsilon ' \approx \epsilon$ and $t' \approx t$ exactly? Is there any specific criterion? (e.g. $\epsilon' \approx \epsilon \Leftrightarrow |\epsilon ' - \epsilon| \leq negl$, or something else).
I cannot find rigorous mathematical definitions about reduction tightness.
Thank you in advance.