In highschool, or for a quick estimate I would accept the (Ka * x)1/2 method.
I have no idea about American HS, but I doubt at any level you are asked to remember just a formula without understanding anything (unless American HS has gone so low ).
Whether this approach gives correct results or not depends on whether some approximation holds - and it can be easily checked if it holds.
(Shane: I am more then sure that you know 5% rule, I just can't agree with the approach of accepting the method in HS).
The problem is, most American high schools have gone that low.
In fact, the majority of undergrad colleges now seem to be going that route.
In high school I would focus more on concepts and theories and less on computational aspects of chemistry my self. Plus, most Americans suck big time at math, unfortunately, I am not sure if the average American high school student knows the quadratic formula.
That is not to say that all are stupid, many come out of high school with enough knowledge to skip entire freshman level college chemistry.