re: software, that's ok. You can still do the same thing in terms of figuring out the bitterness levels by hand.
The point is to work the total ibus from the late additions back to the early additions. You can "artillery" your way in to selecting the amount of bittering hops you need in order to hit your desired target once you have taken the flavor and aroma additions into account. How you actually calculate those numbers is up to you.