Just set it to the highest level.
FLAC is lossless compression regardless of what you set it to, but if you set it to the highest compression level it'll take a bit longer to analyze the data and see if there are a few ways it can compress it better to make a smaller file without losing anything. Audio encodes quick enough that you can just set it to the highest level to try and keep filesize down and not have to suffer waiting endlessly for compression to finish like you have to with video.
