Since i normally drawn on digital i usually use lines to measure, indeed on a separate layer, it's completely normal to drawn on the wrong layer, i do it all the time, but since it's only studies it really dosen't matter, with practice you do it less tho, and if the wholle layer thing is getting on the way just don't use it, the less things in your way the better, if you want it just eraised it later, and with the pen meassuring thing, when i'm struggling with it, i use another pencil to measure with my other hand, so i don't miss the mark, what really metters is the fist line tho, try to chose a line in the middle of the figure, try to split it in half, that's the most important mark, it will be the line of referrence for the whole drawn. If your hand is shaking try to put your ear on your shoulder, make that arm as straigh as possible and hold it with your other hand. I hope that help you, and with the whole zoom out tip your lines looks way more confident, keep it up. Sorrry for the huuuge text btw XD.
