It seems every contractor has their own favourite way of marking blocks for cuts. Some measure using a tape, which we find time-consuming and slow. Others have an adjustable profile guide, but these are less than accurate and again, slow down progress. The method we have used for 20-odd years, and the way we teach all our trainees, is to align the block in position over the gap, learn to align your eye directly above the cut line, and then use the edge of a shard of slate to mark the block 3-6mm short, so that, allowing for the joint widths, it drops in the gap neatly.
It really is just a matter of practice. Some lads and lasses pick it up in an hour; some can be at it a week and still not get it, but that's human nature for you.
I had to amend some paving on my driveway a couple of weeks ago, to make rooom for a new rose bush, and so I took a couple of pics as I was marking up the blocks for cutting - I had to get my son to cut them for me, as my knackered spine won't allow me to operate the splitter any longer, but at least I've not lost the knack of 'eyeing-in' the cuts. :)
Here's one of the pics that was/is going to be added to the 'Cutting Blocks' page, as soon as I get the time.