Making a lower quality copy of a file to save a minimal amount of space is a waste of my time. Besides 2TB is enough space for over 7,000 hours of basic 44.1kHz 16-bit FLAC listening.
Edit: That's what I get for using a random online calculator. Based on the 39,354 FLAC files I have (of various encoding qualities), totalling 1.19516 terabytes and about 9686530 seconds of audio, I can expect a 2TB SD card to fit somewhere around 4,444 hours of FLAC audio. That still seems like enough.