Okay, well 66lbs is 29.937Kg. If you multiply the amount of medication which is 20mg by the amount of kilograms which is 29.937Kg, that equals 598.74mg. 598.74mg divided by 8 hours= 74.8425mg. According to my calculations, 74.8425mg every 8 hours. You might want to round my calculation to the nearest tenth as well. I hope I am correct.