Well those are the type of questions this section is for. You won't find any specific rules which kind of covers your question as well.
The kidos usually work with a range. If you have really low control of kido you probably won't be using anything past twenty. Low control is up to 30. Decent control is up to 50 or 60. Strong control is up to 70 or 80. And if you are kido MASTAH then up to something in the 90s.
Currently there isn't a way to increase your range. Maybe in the future.
There isn't a specific limit. Just know what limitations make sense. You aren't going to be throwing twenty level 80 kidos. Just think of how much energy a really powerful kido would use up of your energy and keep that in mind. So you could do like 30 of the level 1 kido (of course that would mean you either have aim or are continuing to use a really weak kido
).