believe it or not, we have never actually marked the lines out beforehand; mum tends to do all the clipping (she tried to teach me but i always feel inferior - though i am the mane-pulling queen
) and she just......does it! she'll start on one side, and go as low as she feels she could, then if it looks smart, she'll match the other side up. if it then goes wrong, she's got a bit of leeway to go a bit higher!
however, i can remember a couple of years back, a jet went over just as the clippers hit a whorl.....needless to say, trace clip swiftly became full clip!
on the other hand, i suppose you could use white chalk???
You can use baling twine to measure your lines, then use chalk on their bodies to draw on them. I used masking tape to get straight even lines on both sides this year, and it worked really well.
I use a bit of string to measure where i want it to go to then snip a bit of the hair away with scissors then join all the snipped bits up like a dot to dot picture has always worked well on my grey.