Subtracting Different amount of days based on what option is chosen in two different fields

I have a Due Date Field, in which I want it to look at the Item Type Field, and Task Field and based on what is chosen it will calculate a Due Date.  I started typing this Date Formula but this field will only populate if I choose Video Jet which I am assuming I am missing information in this.  I also will do a check box or button override for certain items that will need to be manually edited.  Any help would be awesome. 


If([Item Type]="Proficiency Testing", If (([Task]="Video Jet"), WeekdayAdd ([Due Date], -60)),

If ([Item Type]="Proficiency Testing", If (([Task]="Labels Applied"), WeekdayAdd ([Due Date], -60)),

If ([Item Type]="Proficiency Testing", If (([Task]="Collating"), WeekdayAdd ([Due Date], -50)),

If ([Item Type]="Proficiency Testing", If (([Task]="Dispensing / Gluing"), WeekdayAdd ([Due Date], -40))))))