Here’s the math presented as a flow chart so you can see the timeline. I made the green candies when the mixture was about half reduced.
I calculate THC content as total mass available because the volume was being reduced the whole time (evaporating the alcohol). We started with 10g of dry sift, I assumed we had an extraction/filter efficiency of 75%. That yields:
10g dry sift x 0.75 = 7.5g cannabinoids
I’m assuming the of the 7.5g 75% again is THC:
7.5g x 0.75 = 5.6g THC
GREEN CANDY
So, when I made the green candy, my total solution volume had reduced to 90mL. I took 10% out (9mL, corresponding the 5.6g x 0.1 = 0.56g) to make the candy leaving a solution of 81mL containing 5.6g - 0.56g = 5.04g which I just rounded down to 5g.
I further reduced the solution to only 12mL, still supposedly containing the 5g of THC.
BLUE CANDY
Down the road, I made the blue candies. This time using the much more concentrated 12mL tincture still containing about 5g of THC. I wanted ~3.3mg per candy, each candy cube is 1mL. The candy recipe makes 300mL of molten candy.
I want a total quantity of THC = 3.3mg * 300 candies (1mL each) = 1000mg
My concentrate has 5g/12mL = 416mg/mL concentration (I rounded down to 400mg/mL)
So to add a total of 1000mg THC, I need only 2.5mL of the fully reduced tincture. The fully reduced tincture is much more concentrated than it was when I impatiently made the green candies halfway through the reduction process. So you are correct, that the math is inconsistent. I don’t think it is wrong, but I can see how it would be easy to get lost.
Anyway the syringe pictured contains 2.5mL of reduced tincture containing a bit over 1g of THC, a drop of blue food coloring and half a dram of blueberry flavor oil (per recipe). A dram is 0.75 tsp or ~3.8mL. Half a dram is about 1.9mL. So the syring contains 2.5mL of tincture, 1.9mL of flavor oil and about 0.05mL of blue food coloring, or 4.95mL total liquid volume. I opted to mix the blue color and flavor in with the tincture rather than add seperately so I could visualize the distribution. If the mix is good, color uniformity should equate to THC distribution uniformity as well. And the candy does seem very consistent.
So the syringe is the THC+flavoring+food coloring only. That got stirred into the 300mL of molten candy just moments before pouring into the molds.