Hi!
20% of something is the same as 0.2 times something. So assuming Val is the local variable pointing to text of where the value is held, and Rate is the local variable holding where the percentage (e.g., 20) is held, your expression would be this:
[[Val * Rate / 100]]
This link explains local variables, if you are not already familiar.
Note that if the user is entering the percentage sign as well, you will need to strip that out.
[[ Val * Rate.replace("%", “”) / 100 ]]
That expression will work whether or not the percent sign is entered.