This is such a tough subject for me and I guess I'm not sure there are any good answers but I sure would like to know some rules of thumb that others use.
Some problems are so tough to diagnose. How do you charge your customers when after a couple hours you think that maybe a part will solve it and it doesn't. So you go back in for another hour or so and you put another part and that does the trick. (This story can be written many different different ways with many different numbers).
Bottom line: You have (fill in the blanks) $480 dollars in parts and four hours of labor. The actual labor on the fix was one hour and the part that finally repaired the complaint was $150. If your labor rate is $70/hour, what do you charge them for parts and labor, $290 (includes 1 hour diag) or $760 or somewhere in between. And how do you pay your tech?