Rather than make some test which goes by cognitive functions, or simply to judge one letter at a time, why not make one which types people the same way people do, with a series of straightforward questions specific to a specific type or temperament. There are so many patterns which can help you quickly/accurately identify a type, and none of them are used in tests(that I've Seen)
Rather than just one function with some vague "Yes/ModeratelyYes/Etc" type question. Using this, you could discern an ISTP from an ISTJ in just a couple questions, rather than asking 4 or more. Of course, the questions would have to be tested on people who already know their types..
For example, you could ask something like
Do you consider zombies to be:
::A serious threat (Selecting this would tally up N's and ISTP)
::zombies? like the ones that eat brains? (ESFx)
::That don't confront me. (SP)
::That is silly (SJ's)
Choose the coolest occupation:
::Fighter Pilot (STP and ENT)
::Theoretical Physicist (INT)
::Disc Jockey (FP's and ENF)
At the end of the test, the results will be tallied, and laid out by probability in a descending order.
From there, it will show the 5 likeliest types, with a short description, and then you can pick which out of that suits you best.