Here is how PoZeTools v0.56 calculated filter coefficients,
given your set of poles, zeros and gain parameter:

1 zero gives 2 coefficients:

a0 = G
a1 = -G(Z0)

1 pole gives 2 coefficients:

b0 = 1
b1 = -(P0)

Here is the algorithm: http://home.hku.nl/~pieter.suurmond/EDU/mt/FilterCoeffCalc.c .

Back to the online pole/zero editor.