The IBUs won't change with aging- how bitter the beer is, what changes with aging is that flavor and aroma of the hops will dissipate over time. You still balance IBUS for Sweetness of the overall beer.
No there's really no calculator for something like this.
If you're making a big beer for aging purpose, then go mad with the aroma and flavor additions, just over load them load them, so that as the beer ages and they fade, you still might get some.
Also how are you aging this? Are you bulk aging it? If so, and you want some aroma...also overload the dryhopping of it before you ultimately bottle. The last week before you plan on bottling, add a large dry hop addition (use a nylon bag to you don't have to rack a huge mess of hops.)