In other words, your client is Alabama corporation, ABC Inc., which has $50,000 income with $10,000 of it coming from a K-1 showing, for example, XYZ Inc. of Delaware had $100,000 of income but allocated $10,000 of it to Kansas.  

Doesn't it depend on whether the other state has multiple tax brackets?  I don't know about Kansas, but let's say they tax the first $10,000 at 2% and the next $40,000 at 4%.  Do they only want 2% on your $10,000, or do they tell you to compute the entire tax on $50,000 and then take 20% of that, because 20% of the total income came from Kansas?  With individual taxpayers, many states calculate nonresident tax like that.  (I'm thinking especially of California and New York.)  And with electronic filing, it's easy for them to ask you for loads of data, even when it's unlikely anyone will look at it.