I advise against doing that. (if someone is doing something bad) it's like "I don't think you should do that" To add to is almost another way of saying and. It's the same as "in addition" "This week, I have two essays due, a lot of math homework, and a book to finish. To add to all of that, I have a big test on Friday!" Allow for is not too common but is still used. I think it's a little formal. "I will not allow for that to happen." Commonly, someone would rather say "I will not allow that to happen" "I will arrive at the airport in ten minutes" I haven't heard "average out at" before To act out can mean two things: 1. to act out a play or a character 2. to misbehave, throw a tantrum "he acted out and hurt her feelings"