For those who are wondering what "System Sid" is, here is the diagram from Walt Harris's book, "Billiard Atlas". To hit a spot on the second cushion, multiply the number for the spot by how many diamonds the cue ball is from point X. In the example, the target is 2.5 and the cue ball (Q) is 3 diamonds from X so the target is 7.5 on the short rail. Note that the short rail (M) is numbered 10 to the diamond, the second rail has a variable numbering that you have to memorize, and the rail you are shooting from (O) is numbered in units to multiply with. Harris recommends renumbering the O rail according to the second rail number, so in the example it would be numbered 0, 2.5, 5.0 and 7.5 starting from X.
If you work out the geometry in detail, the numbers on the second rail are off a little but as listed they are easy to remember.