Helpful hints that might help
Use fluorescent lighting directly over your blank, 40" or so, I use the light reflected on the blank as a guide to center my marks.
Use the sticky masking tape, some are better then others.
I use red wax pencil. Take a razor and sharpen the tip as best you can. This will give you more accurate markings. Try to make small dots.
I almost always draw out my markings on a note pad, using a ruler.
Say you want to do 6 diamonds that are visable on your wrap. Say each center of diamond will be 1 1/2'' apart, include another 3/4" on the ends of the diamond, this is what will be visible and where you'll over wrap. I find its a good idea to do atleast one additional center marking(1 1/2"in this case) on each end because you want your threads on the last diamond to be at the same angles.
Once you start playing with it on paper you can better visualise the length and spacings of the wrap. You'll be surprised at what you can do.
Pay attention to your tension