Any way to have conditional default value in a cell while also can set the cell value manually

weikun 

May I suggest that you back up and describe more fully the context in which you're making this request. The context, what it's all about, why this seems to you to be "the way to proceed."

I ask that because what you appear to be describing could work once, but not repeatedly. Once you've overridden manually whatever is in the cell via a formula that references another cell--once you've manually entered a different value--that manual entry is what's in the cell. Period.

So tell us what the bigger picture is. There may be a way to accomplish the same objective, but via different means, but we need to know what the bigger picture is.