How to split Federal AGI for Ky Filing Separate

We have filed jointly for Federal but will file separate for Kentucky.  That means we have to determine which part of the Federal AGI belongs to each of us.  I was trying to double-check TurboTax and I don't think it's doing it correctly.  Most of our income it's easy to tell whose is whose but what do you do with joint income?  For example, interest or dividends on a jointly-held account.