For anyone making their 'own' free-form HDRI images, here is what seems to be the layout of how the image maps to the 'super-sphere'.  This assumes a default camera position.

Area "1" will be directly in 'front' of the default scene.
Area "2" will condense to an area similar in size to area 1, but 'behind' your scene.
Area "3" will be 'overhead', and area "4" will be underneath.
Areas "5" and "6" to either side.

By using these, you can pre-plan where your major lighting effects will come from: 
Do you want 'noon-time sunlight'? Put a yellowish light in area 3.  Windows or doorways would be in 1, 2, 5, or 6. 
Lighting from the spooky dungeon...from below, in area 4.

This will also help you anticipate the major lighting from hdri's you find out on the web, as well.

Cheers.

RosemaryR
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"Oh, yeah. Ooooo. Aaaaah. That's how it starts.
Then, later, there's ...running. And....screaming."