I've found that how I plan to be wetting will dictate not only the best positioning of my anatomy, but how I approach putting the diaper on in the first place. Normally, I wear to bed and therefore typically wet lying down (usually on my back, but occasionally on my tummy). For these nighttime diaperings/wettings, I put the diaper on while lying down, and I point my member down. My unit fits directly between my testicles, and I tend to tape the diaper snugly for this occasion so my member doesn't shift in the night (since it will be in the middle of the diaper, where the diaper is narrowest, even a 10 degree shift in one direction or the other spells leaks + discomfort.) When I've done it right, this is an extremely comfortable and secure fit for me, and I get the most out of the diaper because the Abena M4s I wear to bed have ample padding in the back. Also, when pointed straight down, I have never gotten even a hint of an erection, but my sexual interest in diapers is secondary for me, so that's likely a contributing factor.
If I'm going to be wearing around the house or during the day (in which case I'll mostly be wetting from a seated position), I put the diaper on while standing up against a wall, and let my member hang naturally. This usually results in it pointing out (perpendicular to my body), or slightly down and forward (45 degrees out). Because of the positioning of my member this way, and the fact that I usually wet from a seated position during the day, I find that I primarily make use of the front of the diaper's padding first, with some wicking to the back depending on the diaper I'm wearing. This works for me because I like to feel the bulk of a wet diaper between my legs when I walk (not an issue when I'm in bed), and I can always adjust my member toward the back after I've saturated the front, and get a few more hours out of the diaper.