I don't use a scale. I mix very close to 50/50. The "B" side is a little heavier than the "A" side.
Let's say you have 1 fluid ounce of "A", I put About 7/8 fluid ounce of "B". And have even made it 50/50.
The only time I have a problem is not mixing it long enough.
Remember, you have 6-7 minutes open time and although in your mind, you have to hurry, if you work steady and don't stop, 6 minutes is a long time.
My rule in the shop--When a power tool is running, I do not look up or talk to anyone until the tool is OFF. The same when once the "B" side is poured into the "A" side. No distractions until it is the pot.
Today with the low cost of digital scales there is no reason not to have and use a digital scale. The postal scales by pelouze read in lb/g. oz/g 0.1oz have a zero button and the one I have had for years weighs up to 5 pounds.
To keep it clean just place it in a sealed gallon size kitchen storage bag.
I also have the 70 pound HF digital scale and the pocket digital scale. They are as accurate as any other brand of import scale you can buy. Or at least they really close to the Post Office scales. Tenths/hundredths of a unit, when I have mailed a package.
The only thing that you need a certified tested and inspected scale in Michigan for is if you are engaged in the retail trade, and selling items by weight.
I have used the plastic bag trick with every scale to keep them clean.
The pocket digital scale is great for carrying around to rock shows, when buying meteors, or other material that is sold by the gram.