Transistor CCS resistor value calculation.

[ppppenguin]

Not sure that this cascode configuration gives you much benefit in the intended application. One important point is that the current in the resistor chain and zener must be much greater than the base currents of the transistors. Another way of putting this is that you must never rely on hFE when designing, except to make sure that the base current isn't going to give trouble at the lowest possible hFE.

The current is set by the zener and emitter resistor as you say. Won't be very stable with temperature but that may not matter. A current mirror would be better https://en.wikipedia.org/wiki/Current_mirror There are also many better current sources, easily found online. But as I said, it may not be very critical in your application.

The total of R1+R2 is given by the supply voltage minus the zener voltage divided by the zener current. You are then free to set the base voltage of the upper transistor to any convenient value by choosing the actual values of R1 and R2.

So for 80V HT and 5mA zener current the total of R1+R2 is about 75V/5mA =15K. The base currents are small compared to 5mA and so can be ignored. If you wanted 15V on the base of the top transistor (and hence just under that on the collector of the bottom one) there would be 10V across R2 and 65V across R1. Hence R1 must be 6.5 times greater than R2. So R2=2K and R1=13K would work. Since the zener current isn't critical and neither is the base voltage of the upper device there's a lot of latitude in the R1 and R2 values. Watch the power disspated in R1. 65V*5mA= 325mW so don't use a little 0.25W part.