50uA current standard 

There's a 6.4V drop from the top of the circuit to the bottom of the zener. At the "input" to the wilson mirror, that is is also taken up by two diode junctions, say ~1.3V, leaving 5.1V. The resistors down the input line to that point are (10k + 2.5k + 91k). Following ohm's law, we can divide 5.1 by that to arrive at around 49uA of current coming down that line, which is mirrored on the output. For "normal" zeners, they usually hold a 0 tempco closer to 5.1V or 5.6V. BTW for the ...next time you are building such a current mirror, I suggest to weld the BJTs right next to each other so you can bend them to literally touch their bodies together. Even dab a tiny bit of thermal compound between and then superglue to hold it in place if you wanna go ham. The diodes themselves aren't temperature stable but their shared temperature keeps the mirrored current stable.