% Complete automatic calculation but restrict the input on the child level tasks


I have a % Complete column right with automatic calculation of % Complete on parent level tasks. This automatic calculation is based on user input of duration and percentage completion on each of the child level tasks. Now I want to restrict the user input of percentage completion on child level tasks to a certain list (such as 100%, 50%, 25% or 0%) instead of any value user want (such as 92.9%). How can I do it?

I tried to modify the column property of % Complete and change it to drop down list (with single select). But that seems to destroy the automatic calculation on the parent level. Now the parent level completion percentage requires user input too, which I don't want.

Appreciate any help.


Sean Wu