Skin. Don't use it on your skin like a quick way to wash your hands. 
If you wanna put it through your PW use a ventura downstream injector. This brings through the chem after the pump downstream of hoseline. Can be adjusted to reduce amount of chem that goes through.
Hypo would rot your pump seals aftertime if sucked through pump.
We've sucked through pump neat for heavy soiling a few times but immediately flushed pump with fresh water.
Each surface ratio mix can only be judged from your own experience based on the extent of the soiling.
Flagstones 1:4 but depends on the type of flagstones and again extent of soiling. You'll learn what's needed when you come to tackle heavy stuff like tough ingrained areas under trees that pressure won't shift.
You'll either use just the right amount and get it spot on, not use enough and redo again or use too much and have it gleaming faster.
We don't get it spot on everytime 'cos every surface is different.
Just be careful with it. Get your eye protection on. Pour gently if lifting the drum or get a hand pump to prevent the tiniest splash. I got it in my eye when not putting the drum down gently causing a little speckle of it to splash out.