In addition to a Starsan spray bottle, I like to have a small 1/2 gallon bucket around, like those from Margarita mix, half filled with Starsan. I keep a couple small cotton wash cloths in there for mopping Starsan around bucket lids, wiping rims, hoses, insides of buckets, around air locks, necks of yeast starter flasks, filling air locks, etc.
I also prefer to have a larger, 3-4 gallon bucket with Starsan around to dunk pieces of equipment in during brew day, racking day, yeast handling, kegging, bottling, etc.
You could use a 10-20ml plastic syringe to dispense 5.6 - 6 ml of concentrate into a gallon of water. A 10% measuring error should not have any impact, given the geometry and precision of their 16 and 32oz dispenser bottle.
Save the working solution you don't use in a jug for next time. If you use RO or distilled water it is supposed to not become cloudy, although I've never had any trouble using it when it does becomes cloudy, which happens usually after 2 days. I use regular de-chlorinated tap water.