im not sure the solubility of thc in alchol so i cant answer how much bud you need per ml of alcohol. but how ever much that is is how potent you can get the oil. If you find that value the potency of 70% and 99.9% would be the same but the 70 would take longer to make the liquid evaporate because water and other minute additives in the iso alcohol take longer to evaporate than the alcohol.