Abstract: Techniques and apparatus are provided for improved diagnostics of downhole dynamometer data for control and troubleshooting of reciprocating rod lift systems. A method for pump fillage determination for a reciprocating rod lift system is provided. The method generally includes measuring downhole data during a pump cycle, wherein the downhole data comprises a first plurality of data points associated with an upstroke of the pump cycle and a second plurality of data points associated with a downstroke of the pump cycle, each data point comprising a rod position value and an associated rod load value; converting the data points to non-dimensional data points, calculating non-dimensional slope values between non-dimensional data points; and determining pump fillage based, at least part, on the calculated non-dimensional slope values.