I use 1 small round blue scoop that came in my first Oxiclean tub many moons ago to every 5 gal warm to hot water. I use this ratio to clean carboys, primaries, kegs, and even clean labels off of bottles. I find that in general the hotter the water the better though. This small blue scoop is almost exactly 2 Tbls or about 1/2 oz on the huge green square scoop in the current package.
IMHO, the current scoop is huge, too big for pretty much everything. I just throw it out and use my little blue one for homebrewing, laundry, and everything else. I guess they upped the scoop size so they could sell more. The product is very effective at much smaller concentrations.