The best answer to this question would be, 'By experiment!' So let's start with an experiment. Grab your hihat and microphone and follow me...
(I'll assume you only have the one mic. Choosing a mic for the hihat is another issue.)
First place the microphone close to the bell of the hihat, within a couple of inches. Now make your best impression of a drummer and strike the hihat a few times with your drum stick. You are recording aren't you?
Now reposition the mic to just above the edge of the top cymbal. Bash the hihat again.
Now listen to your recordings. The sounds are distinctly different aren't they? You could also try a mid point where the sound is likely to be a little less 'cymbally' and a little more 'sticky'. In modern recording techniques it isn't so much what the instrument is supposed to sound like, it's what YOU want it to sound like.
Now move the mic further away, first six inches, then a foot, then two feet, then a meter (a meter is 3.1 metric feet - did you know that?).
Listen to all of these sounds on your recording with the intention of developing your 'ear' for the miked-up sound of the hihat. When you are miking the whole of the kit for real, you will attempt to capture your ideal hihat sound, making compromises as necessary because of the other instruments of the kit.
One point you might consider is to place the hihat mic so that the hihat cymbals are in between the mic and the snare drum. This will give you a little extra isolation from the snare; not much but worth having.
So there you have it - you are now a hihat miking expert!
P.S. It just has to be tried so that you know... Angle the mic horizontally and point it between the two cymbals from close range. It's a learning experience...
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