My $.02:
If you're thinking "To get this 3-railer to come off really long I need it to come off the 3rd rail
here, so I need to hit the first rail
here with a little bit of runnng, to hit the 2nd rail
there," that's a method (mostly feel and experience rather than calculation to find the places on the rail that look right.)
If you're thinking "This diamond to this diamond will equal that diamond on the second rail and from that diamond to that diamond= first diamond on that rail, so I need to aim just inside that diamond with center ball," that's a system (using pre-existing target points on the rails and adjusting off of them.)
My guess is that you only use a diamond to help establish an exact first-rail aim point after you've figured out what looks right for where you need to hit each other rail to come off the last rail toward your target (straightback's "instinct" method.)
Edit: ah, now I see petie said it simpler. (I had all these damn words layin' around and I had to put 'em somewhere.)