If you want software that'll calc it for you, try this freebie. You input your gauge, the number of rows involved and it'll tell you when to make decs. (Use the Shaping option.)
If you want to calc on your own, check out Righetti's book 'Sweater Design in Plain English.' She shows formulas on when to make incs/decs on sleeves, necklines, etc.
With KnitComp you'll often end up with shaping on both the RS and WS whereas Righetti's use even numbers so they're always on the same side.
I've used both and find them equally useful, esp when changing gauge, designing, re-designing, etc.