So each recipe calls for the same amount of oil, but you want to split in half each way. From what you self report, you don't use tons of THC on a daily basis. (Clipping after 1/3 of a joint) So 50mg per slice of cake or brownie will be fine for you. In fact you would probably get reasonably high from 1/2 that. That depends largely on your unique biological circumstances.
But you can just make 1/2 cup of canna-oil with your EVO, and split it 1/4 cup to each recipe. If you do that, you will just need to cut your canna-brownies and canna-cake into the same number of servings. If, on the other hand, you want to cut your brownies into 4 pieces, but your cake into 6 slices, you will need to make 2 different strength canna-oils to maintain the same dose per serving. (My laziness would prevent that! But you do you)
The formula ends up:
- (Number of servings per dish) x (number of mg per serving) = total number of mg needed
- (total number of mg needed) / (number of mg/ml of tincture) = ml of tincture needed
Then, yes, it is as simple as combining the oil and the tincture in a bowl or pan and leaving it exposed to the open air. In this case. the greater the surface area exposed, the better. As it will speed up evaporation. But DO make sure that the de-carbed weed doesn't get outside the oil and stuck to the pan. As for an exact time, all that I can say for certain is a day or less. It depends on environmental factors such as surface area, ambient temp and relative humidity. If you live in a rain forest it will take longer than if you live in the desert.
To be 100% positive, you would really need to measure the specific gravity of the EVO, and calculate what the specific gravity of EVO with de-carbed weed added to it would be, and then measure for that number. But, we're making a damn cake, not trying to land on the moon, so I would say that when the canna-oil is returned back to pretty much the same viscosity as it was in the bottle, you are good to go.
Just stir the oil and tincture mixture every couple of hours or so. It's not some critical timer based thing. In order to make things go even faster, I would mix all the tincture that you need with 1/4 cup oil. Then, when you are satisfied that it's been properly evaporated, add the other 1/4 cup oil and mix it all together really well. Then split it into (2) 1/4 cup portions and you are good to go.
So, as an example, let's say that you decide on 50mg/brownie/slice of cake, and that you will yield 6 portions of "canna" each. 50mg x 6 portions = 300mg/ recipe. Since you are making 2 recipes, that is a total of 600mg THC needed. Your tincture should be somewhere near 8.33mg/ml so: 600mg / 8.3mg/ml = 72.29ml of tincture. That equates to roughly 5 Tbsp of tincture. (4.8 to be exact)
As I said above, I would mix that into 1/4 cup EVO, and then let it evaporate. Then I would add the other 1/4 cup EVO, and mix well. And split it back into (2) 1/4 cup portions. That, at least is, close enough for government work!
I too like rum cake. My problems with cake are so many people make them too dry. That's not really an issue when you add some rum. You could either use canna-oil in the cake, or make a tincture with 151 Bacardi, and use that, or do both! It would be one hell of a dessert!