In our vat we start with 250 gallons or so of hot water then slowly add our products as we bring the total volume up to 500 gallons. The mixer is running to blend.
Hot water first and then put powder in and mix it.
If liquid is being used, the other way around to make sure there is enough room for the liquid. Downside is that if water is being put over chems, it could foam up more than the other way around.