TBH, I just run my finger again the direction of the hair.
I don't think they is a really 100% works everytime way of marking where you are going to clip, as the horse moves, you brush the hair away etc!!
Or you could use a lippy, one of those horrendously bright pink ones that was an unwanted Christmas prezzie that has been lurking in the bottom of your makecup bag for that last couple of years!
For fussy clients I used to mark with soap, and use my hand to get the distances level. Now I just whip it off a bit lower than intended to give myself room to tidy up. Saddle patches, just go round the saddle.
I do the leg lines by freehand just by my eye, but when i do a full saddle patch i put the saddle on girth it up and draw round it with a old lipskick works perfectly every time, this way it doesnt matter if the horse twists round cos youve still got the mark, and you re not trying to hold a numnah with one hand and the clippers in the other. Weve all got some scarey coloured lipsticks lurking in the deepest depths of our make up bag!