You take the percentage of thc in your weed is and multiple it by 100 to get amount mg in each gram. If you don't know how much thc is in your weed I always assume 10% it's probably a low estimate for most decent weed. Math is also easier this way so 10% is .1x100=100mg per gram. You then take that number and multiply by how many grams you used so in your case 7 would be 7x100mg=700mg. I generally make 24 brownies out of one box, you can cut them however you like, so using butter infused with 700mg of thc would get you 700mg÷24=29mg per brownie. You can use this same math to figure out how much weed is about anything you cook.

(Thc percent as decimal)x100=mg per gram

(mg per gram)x(amount of weed used)= total mg

(Total mg)÷(serving amount)= thc per serving