Question About Naming Molecular Compounds

Apr 2018
Okay so I have started this debate with my Chemistry Teacher about how you determine if a compound is Ionic or Covalent. I was basing on how I name a compound based on the Electronegativity. Based on the

χ<0.4⇒ covalent non-polar

0.4<χ<1.7⇒ covalent polar

χ>1.7⇒ ionic

So for example on PbI2 which is Lead and Iodine it's technically considered a Metal and Nonmetal. I went off Electronegativity so Lead's is 2.33 and Iodines is 2.66 so that is a 0.33. So it would be considered to be Covalent Non-Polar.

So if we go off of how she is basing of her method by saying Metal or NonMetal to tell if it is Ionic or Covalent. PbI2 would be Ionic. So we are stuck on this debate on who is correct. So I guess my question is what method should be used when Finding if a compound is Ionic or Covalent.
Mar 2018
My chem text says that if the first element in the formula is a metal, than the compound is considered ionic. Is this how she is basing her answer?

I can give a reference if needed