The thing that would help me an awful lot would be knowing how GW rounds casting times... Things like damage and energy are rounded to the nearest one, but what is it for casting? The smallest innate gap is 1/4 of a second, but does the engine keep a .2 or .15 second cast as calculated?
If you take a nice, 1/4 second interrupt, throw in some fast casting, and mix up some coffee (optional), you've got .2 seconds to catch a spell, plus whatever your reaction time is: given the standard of error on most tests mine is usually at or around (trending under, often) .2 seconds - so all told you've got to expect at least 1/3 of a second for an interrupt to trigger, give or take lag. If you're jittery you can sometimes even anticipate a 1/4 second cast and start your interrupt about a reaction time's worth before the cast, thus interrupting the 1/4 cast, but you definately can't do that at will. So all things being equal (by which I mean your reaction time being <1/4 of a second, your interrupt being <1/4 of a second, and your target's cast being ≥1/2 of a second) you should be able to interrupt almost every time.
It usually helps to take some online tests or something, to give you an idea of where you stand on reaction time.
I got 45 on this one.
I got an average of .175 here, an average of .198
here, and an average of .196 here. In other words, today was an off day.