I want to know how to express the result of reduced liquid.
There is no single answer to this; it depends on the context and the intent of the recipe in which the reduction is being performed.

Most reductions are expressed as a very approximate percentage, such as "reduce by about half" or "reduce by about two thirds."

Sometimes, you may specify a target amount if that is what is important.  "Reduce to about 1 cup."

Lastly, sometimes you specify the outcome desired, as in "reduce until thick and syrupy" or "reduce until the sauce coats the back of a spoon."

Your specific example of five quarts to three cups is very extreme.  Probably you want an outcome based description, not a percentage.  When specifying by percentage, it is usually for more basic sauce reductions such as a pan gravy after deglazing.  Still, the way you have written it is perfectly clear; there is no reason to calculate a percentage if that is your actual intent.