I have made rosin-based tincture/oil for my son for epilepsy for several years.
After pressing the rosin, I decarb it in a small mason jar in a sous vide bath at 204*F for 90 minutes.
Then I use a heated lab plate with a magnetic stirrer to mix MCT oil into the rosin. This can take some time, patience, and silicone spatulas to help mix. You want it very thoroughly combined until you have a uniform-looking oil, all the same color (could be very light or amber, depending on your rosin color and your final concentration).
To figure out the dosage, I decide what strength of tincture I want. I make 10mg THC/ml.
This is approximate, but I figure 60-70% THC in my rosin. I have two strains I had tested and one came up 57% and one 70%. So there could be a bit of variation in strength, but not a huge one.
So for example 1g of rosin has *roughly* 0.7g THC, or 700mg. To make a ~10mg/mL tincture, I mix that 1g rosin with 70mL of MCT oil. Yes, the rosin itself has some volume, but it's not an exact science, just a way to get *close* to accurate dosing. (and it's only a few mL of volume)
Now you have a tincture where if you want a 10mg dose of THC, you take 1mL of tincture.
I store it in the fridge and it lasts at least 6 months, potency will slowly degrade over time after that.
He just takes it by mouth via syringe, doesn't try to keep it under tongue, but the MCT oil and the method of making it seems to help it come on in 20-30 minutes and last about 4-5 hours more like vaping than eating.
If anyone tries this recipe, please let me know how it works for you.