How to calculate the mg of the in your edibles

The reliable commercial edible makers get their edibles lab tested.
I once ate a 100 mg dispensary rice krispy treat and felt nothing, so some makers don't test accurately.

The amount of THC depends on the quality of the shake, which can vary between stronger than flower (if it contains lots of fallen trichomes), to about half that of flower (if it's really just dispensary trim).
Assuming 15% thc means that you've got 15 g = 15,000 mg thc to work with.
If you don't strain the oil, then the amount of thc per mL is about
(15,000 mg thc)/(4 cups)*(1 cup)/(235.6 mL) = 16 mg thc/mL
That means 80 mg thc per tsp, or 240 mg thc per tbs.

If you strain the oil, divide these numbers by 2, because half the medicine will end up in the strained out sludge.
Make sure you decarb first (240 F for 40 minutes with a verifying thermometer), and then grind the herb to powder.
Simpler is to just decarb, grind, and mix into the oil of any recipe, or mix with nut butter and use. This is my favorite method, but I don't mind slight herb taste. Last night I had some 17 gram mini-muffins that contained 0.25 g herb each, and I tasted no herb. When I mix the same amount of herb with 4 g nutella, the herb taste is strong.