Best frequency range to measure Khomus fundamental pitch.

Curious to learn if anyone else has a pre-existing opinion on this:

I'm making a record currently with various khomus instruments that are not on standard pitches by tuning sound sources to mathematically derived 'custom' temperaments. The basis of this rests (of course) with a precisely measured fundamental pitch. I'm going to filter the recordings in order to eliminate confusing overtones, I've been doing it 'by ear'.

Does anyone else have an opinion on what would be the most effective frequency range to measure the fundamental inside of?

Best,
Oscar