What body style of guitar? Some are prone to neck dives because the bridge is so deep into the body and the strap button cannot be on the neck (eg., 335s, SGs). I recently got an oversized SG-style, so without any positioning, the 15th fret on that is where I am used to the 12th fret being for the same scale (25.5"). For standing, one solution I have not tried is attaching the strap like an acoustic (not to a button, but with a string at the nut), but what I have been doing is hiking the guitar higher and resting my forearm into the top notch so I can use my right arm to hike it roughly 3" to the right. I find that I will also tip the neck outward a bit so my left hand doesn't need to "reach to the left" so much.
Sitting is similar, the body notch on an SG is beneath the bridge more than other guitars, so right leg position can help pull the neck 2-3". I also find the right forearm use similar here, and will use that to hold the guitar right, when my leg is actually riding up the lower horn.
Not sure how much help the above will be, but the style of the guitar will affect how it can be carried, and there are some straps out there specifically designed for select functions as well.