There are many possibilities due to the number of variables. The type and size of room, distance to microphone and type of microphone are just a few. You might chose to use a valve microphone to make the sound warmer or a ribbon microphone which tends to give a rounder sound at higher frequencies. It's really important to have a good room which doesn't accentuate unwanted frequencies. Microphones too can accentuate frequencies so it's useful to have a microphone with a good 'flat' response. As a starting point try placing the mic about a foot away from the whistle and slightly offset i.e. not directly in front of the instrument. It is purely down to what sound you are looking for and as such experimentation is the key.