The perfect seam allowance is something we all work at, pretty much all the time. We each have to find what works for us and there are many solutions. The big thing I think is consistency, some times you are going to run a little big or a little scant, or maybe your machine makes that decision for you, but you learn to work with what you have.
I prefer to press open, I feel I get more exact matches, but I also have to pin the heck out of everything. I also am a fiend on prewashing and pressing. So much pressing and I frankly,dislike ironing for the most part. I have friends who press to the side if they press at all and never prewash (ok, usually I can tell -- but sometimes they get tremendous results) I like to do all of one step at a time, so if I have to make 1000 HST for the top, that is what I do. Other people go block by block. I used to be all about being very very precise, gave that up and the last ten years I've been riding on the cut/sew large/trim down for accuracy bus. It may be a fun and fast bus, but there is still a lot to be learned on just how to do that.
I have and use a lot of tools and gadgets to help me, but I did a lot back in the day without more than basic supplies like folded paper, plates (and cans for circles), and string and a straight edge.
Fabric can be treacherous, it wants to move and shift and stretch. We want it to do what we tell it to do. It's like the joke about the mule, sometimes we have to get the fabric's attention!
You are doing extremely well for just starting.