Personally, I wouldn't worry much about which lead to post on when learning to post. You can pick that up later. And out on a trail, where you are not always turning, you just post - no lead to worry about.
Ideally, the horse's motion will throw you up & forward some. I tend to think of it as up and forward, and then straight down because the horse has moved forward underneath you. Starting, I would put a lot of weight on the stirrups and make a conscious effort to keep my knees apart. With time, you can kind of roll onto your thighs instead of putting so much weight into the stirrup.
If in doubt, I'd start in two point to get my basic balance and position. Once you feel in balance and synch with the horse, start sinking down lightly into the saddle while maintaining the basic position. That will help you to avoid doing what my youngest daughter was doing in the below picture (taken a few years back...she doesn't ride like this anymore):
Back then, she started in a severe chair seat, then gripped with her knees and shoved her hips forward and up while her feet pivoted back. She was never balanced over her feet to begin with, so she stayed unbalanced in the post. The picture below is from the US Cavalry manual and shows a canter, but the idea works for the trot and posting as well - the weight is balanced in front and behind his stirrups. Since he is using a forward seat, his heels are slightly in front when seated:
I also think of posting as a discrete thing. Once your weight is out of the saddle, extra height doesn't help. The purpose is to get weight off the horse's back, not to fan our crotches! My goal is to get my butt off the saddle, but leave my jeans in contact. The less I go up, the less likely I am to fall down hard on my horse's back, which defeats the whole point of posting.
All FWIW. If it doesn't help, ignore.