I know a question on adding Flying Press was already suggested, but just thought I'd throw in Freeze-Dry too, since it hits water types super-effective. It could just be a little tick box under Ice type to click if it is Freeze-Dry. I know its a very basic calculator but it can save a lot of time for others.
I think this would be alleviated if using the formula/script that shows the sprites (which I'm guessing it does by look at type combinations) was displayed in a type chart. Currently, if you select a type on the type chart it shows how a move of that type would do vs any other combination of pokemon, but it would be nice if you could select multiple types and have them show up in a combined chart. Like if I pick Electric and Ice as move types rather than list all the pokemon, in a chart it would show that electric/steel, electric/fire, and electric/ice can't be hit better than "Not Very Effective" (1/2)