正好【zhènghǎo】

正好 before a verb says something happens to line up with something else – "as it happens", "just then". It goes after the subject.

你来得正好 is a fixed greeting: "you've come at just the right time".

Just right

After a noun, 正好 is the comment: the size, amount or temperature is exactly what's needed.

Before a number, it means "exactly".

刚好 means the same in all three uses and is a little more casual.

Where it goes wrong

正好 is already "exactly right", so it takes no 很 or 非常.

• × 这双鞋很正好 → 这双鞋正好 / 这双鞋很合适

It is about coincidence, not about something in progress. "Is doing" is 正在.

• × 我正好吃饭，你等一下 → 我正在吃饭，你等一下