Did you know that SharePoint supports list validation in addition to column validation? I’ve seen the option 100 times before but it never clicked before I actually needed to use it. In this case, I needed to make sure that anyone setting a task item Status column to Completed” was also filling setting the % Complete column to 100%. Or, alternatively, if they were setting the % Complete column to 100%, they also set the task Status to Completed.
Need to do the same? Here you geaux:
=OR(IF(AND(Status<>"Completed",[% Complete]<100%),TRUE,FALSE),IF(AND(Status="Completed",[% Complete]=100%),TRUE,FALSE))
Note that this simple formula doesn't care about the percentages of any other status. If you'd like that functionality, check out the solution on this TechNet post.