I've had luck printing out a photo, then figuring out how much bigger it's going to be on the drawing to find a factor to multiply by, then measuring the photo with a ruler or caliper and multiplying that number to get the bigger size. That prevents me from getting too far along and realizing I've got one area that's out of scale. It's kind of a backup to my eye, because as I go along, I tend to sketch things in, and if I measure them and check it, I often find it's about right.