Fast formula to set default element input value

Hi All,

I'm trying to set up an element entry input value defaulting rule. I've seen different examples, some very simple others more complex.

I'd need a formula that considers more attributes but, after many attempts, it keeps giving no results. I've also tried an extremely simple form as follows:

INPUTS ARE Perc (number)

DEFAULT FOR Perc is 0

Perc = 20

return Perc

Perc is the name of the main input value of the element. Earlier I used "entry_value", as seen multiple times around. What am I doing wrong? I notice that, whenever I manually change the value of the input value from "Element entries", the system loads something, so I assume that somehow the rule is running.