It took me a good hour and lots of erasing and re-drawing to get the proportions right on this, which I made from my own photo. Even though the photo was a square format, I ended up making the tiger too large and therefore losing my negative space. I did find the measuring useful, though trickier to use with a photo. And I still managed to get the right eye (our left) in too close initially and having to correct it. Same thing with the chin, which was initially too small and needed correcting. So a modicum of perfectionism (not my strongest point) is a good idea here!
Interesting observation: I always think of tigers as being orange and black, even though I spent almost 2 weeks photographing them in the wild last year. Now, when I started adding color, I realized how much white they have.