I suppose it’s how limber you are…
TIP: HOW TO FIND RAFTERS: MAGNETS!!! Any neodymium magnet, like from a computer hard drive, will strongly stick to a sheathing nail - they will stick even though arch shingles. If you want to have fun, get some neodymium ball magnets and you can get the precise nail location, in fact all of them on the rafter which can help when encountering sheathing end nailing.
One way is to shingle as far as you can and then put in a row of jacks. Problem with this is the next shingle course will be at your toes as you cover the jack ends. I have to sit on the plank to do the next couple courses. Alternating between standing an sitting is a Royal PITA.
Other way is to put jacks down and then shingle over them at least 2 rows before working on them. Issue with this is that your jack rows are now closer by the number of courses above you shingled.
I’m doing 7 in 12 pitch which is 30*. The plank is 2x10 and the jack is 45*. To get a level plank, I lay a 2x4 underneath at the roof end or the jack.
I usually use 2"x6" 60 degree jacks if I am on something unusually steep I will break out the adjustable 2"x10" jacks.
I usually space my jacks about 5’-6’ apart going up the roof.
I find trusses/rafters by banging on the roof looking for a solid spot the get the first one and then measure from there.
On a 7/12 I would just put jacks at the bottom and walk the rest.
I can walk the 7/12, but my ankles are very unstable - I can walk on the sides of my ankles! And I’ve had an ankle roll out from under me occasionally. A tightly laced up hi-top boot helps.
But at that pitch, everything slides off the roof including me if I want to sit or lay on the roof (I’m 5’9 and 280# so that’s an issue too.)
FINDING RAFTERS: Magnets. This is a 1930’s house and I wouldn’t bet my last dollar on the 24in spacings. I did 5 jacks and using the magnets, zero misses. (I do computer repairs, so I was using a hard drive magnet.)
Easiest way to find rafters, if you know you are going to be installing jacks, is to take a keel and throw some marks on the underlayment as you install it.
Did more jacks, tried the bang with hammer and listen - man, that is a No Brainer. I have 1 by sheathing which I think is even easier to hear than plywood.
OK, what’s a keel… (off to Google.)