I seriously doubt anyone has created such a calculator, or posted it online. Any such calculations that have been done were probably done on a case-by-hypothetical-case basis by the particular author.
Some main factors that would determine the atmospheric effects of an impact:
- Relative velocity of impactor - An asteroid moving along a similar orbit as Earth, in roughly the same direction, would be less energetic than one which crossed Earth's orbit at a 90?° angle, or worse, one that hit the planet head on.
- Size/Mass/Density of impactor - Is it big and heavy, small and heavy, or big and light?
- Composition of impactor - There are three main types of asteroid: silicates (rock & metal), carbonaceous (carbon compounds), and nickel-iron. This is probably less important than the velocity and mass, since the vast majority of stuff kicked up into the atmosphere belongs to Earth already.
- Where it impacts - Deep ocean, shallow seas of continental shelves, or dry continental crust. Again, most of the junk blasted into the atmosphere is going to be of Earth origin, so this can really affect the outcome. Does it hit granite, sedimentary sandstone, shale? How much water is going to be flash-boiled?
- Wild-cards - Any number of other unforseen variables could affect the climate in such an impact. Some theorize that the one which hit the Mexican Yucatan peninsula and killed the dinosaurs splashed molten searing rock all over North America, setting massive forest fires that might have burned for weeks, dumping megatons of carbon into the air and removing a biological sink from the planet. Whether that would make the situation worse, or just add insult to injury though, might be hard to say.
If you're looking for some educated people who might know something about a calculator for this, I'd email someone here:
http://www.llnl.gov/planetary/
It's the web page for the Proceedings of the Planetary Defense Workshop, at Lawrence Livermore in 1995.
(Oh, and you might see if your local library has Rain of Fire and Ice, by John S. Lewis, for a very enlightening treatment of this subject)