Bcache writeback_percent max value

I'm tryng to set writeback_percent at a value > 40 but it only accept value between 0 and 40. If i set echo 50 > /sys/block/bcache0/bcache/writeback_percent then when i read the value more /sys/block/bcache0/bcache/writeback_percent i have 40. For value<=40 the settings work fine. My setting for cache type are more /sys/block/bcache0/bcache/cache_mode writethrough [writeback] writearound none I know this is dangerous but this is not a problem for me. As far as i understand writeback_percent is the % used from dirty data in cache, why i can't use 90% or 100% of available space? May be i dont' understand quite well this settings?