I choose a point on the horizon, far away. I zoom the camera as much in as it can. With the helmet on my head, I ask a friend to point the camera (and my head) to that that point. To judge this, a live view from the camera is needed, or a few seconds delay on the gopro app is also OK. With the app you could probably even do it on your own.
Then I adjust my sight for the same point on the horizon.
My sights are aligned for infinite distance; but since I know the distance between my lense and my eye (10 cm or 4"), I compensate by pointing the sight 10 cm below the point I want centered. So if I want someones eyes of in the center of the picture, I aim for their chin. This works for every distance.
And what also helps: with a new setup I often spend a few minutes shooting pictures around the house, trying to frame the pictures and later check how far off I was. You learn a lot that way. Just wait till you are home alone for it, 'cause you look rediculus doing this ;-)