Advise on how to do a lookup/calculation on two fields

I swear I have seen this on documentation somewhere but cannot find/recall it. 

I have to make a calculation which requires two fields . Field Name 1 =  Source , Field Name 2 = Position 

If Source = Stock , then 'Stock'
If Source= "Process" , then "Process" 
If Position = Upper Embroidery & Source= Purchase , then "Embroidery"
If Position = Lower Embroidery & Source = Purchase , then "Xtra Embroidery"
else if
If Source = Purchase , then 'Purchase', 

Anyway to avoid 'calculations transformation' ?

Looking at it now

I have created a working example for you please see attached

Hello, 
this works !!