A very good trick for getting an accurate drawing before you spend hours on an oil painting only to realize your initial drawing was way off: Do the drawing on tracing paper, then pull out that page and put it under the next page. Trace the lines that are correct then remove the page and do the drawing again. Repeat this process over and over, maybe 5 times or more if needed. Each time you should be able to tighten the drawing and get much closer to the correct proportions. This is the 3rd try at this drawing, the first was not nearly as close but I like this one. Actually the second and third were very similar, its a great exercise and you will get better as you do it.
I plan to paint this soon(I hope).