100" is the current transformer primary current, and "0.050 × 18" is in fact the voltage across the burden resistor for the standard c.t. and burden at that current, so to arrive at your current constant you simply substitute your transformer's rated current in place of "100" and the voltage it gives in place of "0.050 × 18". For example, the Yhdc SCT-013-030 gives 1 V at a rated current of 30 A, so for this transformer you have: