Why can't you do basic operations AFTER an aggregate in a window function? Like (max(x)+5), vs (5+max(x))
If I'm understanding your question correctly, you're trying to divide the result of the window function by 5. The entire window function consists of MAX(x) OVER () therefore your query would look something like this:

SELECT MAX(x) OVER ()/5 FROM generate_series(1,10) AS t(x);


This will result in 10 rows all containing the value 2, since the generate_series function results in 10 rows and each would contain the result of our window function.