First, figure out if it's mental or physical. Then figure out if it's easy, average, hard, or very hard. Cross-reference on the table below to find out how many points, total , you need to spend. In this context, mental skills are not limited by the physical body, and physical skills are. Broadsword is DX-based and physical.
Chemistry is IQ-based and mental.
Sex Appeal is CH-based and physical. Erotic Art is CH-based and mental. Halve for each level of Eidetic Memory you have. People with the Book Memory talent can carry books around in their heads to study from. Maintenance is free if you're concentrating.