Sauce thickener for large batch
The method I was taught doesn't care about the absolute amounts: First add small amount of the sauce/soup/whatever you want to thicken to the 'powder type' thickener and stir, adding more sauce until it's a very thick liquid without clumps. Only then pour the liquefied thickener with sauce into the 'bulk' of sauce. A very modest amount of stirring will distribute and dissolve smoothly and once it heats up and starts absorbing more water the thickening process will begin for good.
If a lot of the thickener stuck to the bowl in which you were mixing it with sauce, transfer more sauce from the pot to the bowl and dissolve whatever stuck to the walls, then pour back into the pot.