Design of Parks-McClellan Lowpass Filter
The minimal specification for a FIR lowpass filter designed by the Parks McClellan algorithm is the passband edge, the stopband edge, and the filter order. You don't need to specify passband and stopband ripples; they are the result of the design process. What you can do is specify passband and stopband error weights, which let you specify the relation between passband and stopband approximation errors (a larger weight reduces the error).