Run a 14'6" string line from top landing to where next level landing will begin. This line represents 1/2 of total vertical drop. Use a line level to determine horizontal height from top landing and measure 14 1/2" down. Bottom end of string line should be supported to this height. You will match the slope of the modules to this line.
Remember that decking adds 1 1/4" to the vertical measurement. If you place a piece of decking on the landing where the string line starts, be sure to use decking for the entire run of the ramp.