Putting numbers on locomotives has always been a painful process for me. Get the string wonky and it will stand out like a sore thumb when you display the model. Of course it doesn't show when you are poking transfers around.
A trick I picked up from somewhere is to use a bit of clear plastic as a guide. The one in the photo is a bit from the packaging supplied in a shirt. I originally had it tall enough to set the crests up and then cut down the hight to do the numbers. A couple of marks on the sheet help the lining up on the body.