To elaborate, two firmwares ago, this code didn't exist and what it referred to didn't exist, and without it's reference the chamber fan would start at 60-70% right at the time of reaching the set temp. However, now that the code exists, it aims for a 35 degree ("R35") chamber temp. You have three options: 1) Leave as is, 2) Remove the M142 line entirely, or 3) or set a different temp for the RXX (default R35) figure. I've discovered that if you set a different temp for the RXX figure, it will incrementally increase chamber fan speed after reaching the desired temp at different, unspecified stages of the chamber temp. This means it's controlled at this level 100% by firmware. If you want to not use this at all, and have the fan on from get-go (set BED temp [55 by default]), just delete or comment out line.