i would say your neckline is most definitely in need of straightening up and you have lots of room before it will be too short. start by locating the top of your adams apple and leave about a fingers with above that. This is should be where you start. Your shorter side should be fine for now and I would work on making the longer side match that. Again start from that center point 1 finger width above your adams apple.

what I did for my beard was take profile pics then draw on them using software and played around with different lines. found which I liked best and did that. Keeping it a little longer is not going to hurt but going too short will be problematic.

Make sure you never cut wet / damp beard. Have it be fully dry to avoid cutting too much off.