not being able to level your seat completely is an issue with cheaper seatposts. yours, like mine have fixed notches in the clamp that don't really allow super-fine adjustment.

i do not know which seatposts offer complete adjustment of the tilt angle. i'm sure there are some more expensive posts that still have these awful notches in them as well, but i haven't done any research on them, since i was actually able to get my angle dialed in with my OEM seatpost.

luckily, there is a 'be-all end-all' seatpost that you can't go wrong with. it's expensive though. Thomson Elite, pretty much everybody who has ever used one will swear by them forever. they're around $70 though, but they offer infinite tilt adjustment, no 'in-between'.

as for your question regarding length, most manufacturers measure their posts from the center of the seatrail clamp to the bottom. you've also got to take into account the minimum insertion mark on each post. just because a post is 300mm tall, does not mean you have 300mm of useable post.