Hey ! Since you talk on proportions i focused on that. it's not pretty accurate on my paint over, but you will get the idea behind it. While continuing the painting, you shall keep a sketch layer with the head construction, including the whole head. So you won't miss the skull size for exemple ( the hat getting smaller that the head proportions don't fit). I advice you to search for the loomis 3D model on artstation, so you can find the perfect angle you are looking for to understand how the proportions should work and helps you to place the face's features. Think in 3D, and since your face is in 3/4 you shall bring the eyes closer and a bit smaller, think as the eyeglasses temples, they are not in the same plan of our eyes ( not sure if the idea is clear there but it's kinda a self tips) and the other major problem is the mouth being far too low, try to mesure your own face with your fingers, the upperlip is closer to the low part of the nose than the bottom one to the chin. hope it helps !