Alright, I'm not fighting that NP may be an ISTJ, I'm just going to give more information because I'm not fully convinced.
The only other TJ actress that comes to my mind is Jodie Foster. I really don't see that many similarities between Portman and Foster. And as someone already mentioned you should look at her film choices. Her film choices don't strike me as ISTJ.
I agree that she is practical and conservative but I don't think those are traits only found it ISTJs.