I use it all the way for all my BW fixing purposes, dilute it 1+7 with water (I find it too strong when diluted 1+4), and I fix about three minutes. A clearing time plus the same amount of time, an usual rule. I always use an 2% acetic acid stop bath before fixer, just to contaminate it less and to stop the development - you see, C41 fixer is ALKALINE.