[Steve Watton]

Depending on how advanced of a course you're taking, your text should have a series of equations and tables that allow you to calculate delta's by incremental additions and subtractions of various shielding and deshielding effects (they should be pretty clearly marked in your text). That's pretty advanced, however. More usual is just to recognize that the chemical shift falls into a range - between 7 and 8 means it's an aromatic C-H, for example - and not worry about the precise numbers. Even advanced users don't use precise values of chemical shifts much - there's usually plenty of other information available to allow you to solve a structure.