Any astronomers around?

[HP67]

I know there are some heavy-duty mathematicians on the forums. Maybe you can explain this also.

I do not understand how to calculate the time of x degrees after some celestial event. For example, let's say sundown occurs at 19:30. How do I figure 5 degrees later? I read that every degree is equal to 4 minutes regardless of latitude. So later should be a simple matter of sundown + (5 * 4) minutes (20 minutes after sundown). But my results don't match up.

Can anybody fill in the blanks? Thanks.