it is accepted that resin can take 5-10% of its weight in additives ie dyes micas etc It’s also said that 1 gallon of epoxy can accept 3-4 ounces of mica Following that theory 1 gallon is 3.78l x 1.16 to get the weight = 4.38 kg 1 ounce is 28.3grams x 3 or 4 = 84.9 grams or 113.2 per gallon/3.78l To take it a step further 8 gallons = 30 litres approx So you’d use 679.2 grams up 905.6 grams for 8 gallons I’d err on the side of caution and add less to ensure it doesn’t affect curing etc