I use a scale. 1# accuracy, or roughly better than 1 pint. I don't use a venturi, I just plumbed them into the suction side of the pump with tees and one way check valves. Moving Bicep II Magnum at 2.4 qt/10 gal solution is much better with a 3" pump.
Pallet scales are $1,000 to $3,000, depending on how fancy you want and how much shipping.
This is the computer age. There is this marvelous thing called the world wide web that has all kinds of information, including the specific gravity of any herbicide you wish to spray. I can look it up on my phone, and then, also using my phone, calculate how many pounds of product I need to put in the spray tank. Water has a specific gravity of 1.0, and weighs 8.34#/gallon. For Example, Roundup Ultra has a specific gravity of 1.17. So Roundup Ultra weighs 1.17 * 8.34 = 9.76#/gal. A different chemical will have a different specific gravity.