Student Grade Tracker - If statement help!

I have created a tracker to track student grades, the tracker works by the teacher inputting 3 grades which is added up and divided by 3, a function then rounds it up and finally an if statement works out if the student is under performing, satisfactory, good etc.

Now my problem is that some teachers want a manual adjustment column adding so they can input their own final grade if they do not agree with the average grade that Excel has worked out.

My question is, can the IF statement work off the average grade and the manual grade at the same time?? For example the If statement would perform its function based on the average grade if the manual adjustment column is left blank, otherwise if not blank work out the manual one instead??

I have attached a copy of the sheet.

Appreciate your help

Stu