Ooh can't wait for this
When I did my Corvette I found the easiest way to do the waterline was to just simply get a looong piece of thin tamiya tape and just sort of go for it lol, stick it where you think you want it and it tends to be about right, but using a single length of thin stuff makes it easier to adjust the position once it's down, but also actually makes it easier to get a straight line in the first place.