The reason they suggest pre-mixing is because when you use DG's and DF's like Celsius, Metsulfuron and others, the tendency is for it not to mix well without excellent agitation. I use a 32oz measuring cup to "pre-slurry" my dry material with the use of the spray wand before adding to the tank. Then of course, you need to recirculate the hose line when you are finished adding products to the tank for the finished mix.
"The Poor Fish" circa 1930's: The Poor Fish wouldn't have been caught if he'd known enough to keep his fool mouth shut.
"Opportunity is missed by most because it is dressed in overalls and looks like work"- Thomas Edison, businessman/inventor