I generally follow these two guides:
How to set up road bike drop handlebars — tips and guidelines - CyclingTips
Road bike handlebar buyer's guide: width, shape, material, setup - BikeRadar (set up section)
For reference, I use the terminology here (
Drop bar hand positions | BikeGremlin) to describe the different sections of a drop handlebar.
On my (one and only) road bike, when the fork and stem are pointed straight ahead, I have the end of the drop just about parallel with the (sloping) top tube (of the bike), in order to (a) provide good support when I am using the drops, and (b) avoid my bar ends hitting the sides of the top tube if the handlebar is turned all the way. I have tried the rule of thumb of pointing the bar ends at the axis of the rear wheel, but that (a) makes me feel like my hands may slide off the end of the drops, and (b) increases the effective reach of the handlebar and puts the hoods too far forward. Then I set the hoods far back enough on the curves to ensure a smooth transition (i.e., slightly increasing slope) from the ramps of the handlebar onto the rear ends of the hoods while keeping the brake levers within reach when in the drops.