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It depends what you want for line regulation, load regulation and range of V, I P.

Do you have any ideas on specs?

This design is sub-optimal for many reasons, but you want to learn analog design.

e.g.

1)  TO-92 Rja= 200 'C/W  with a 12V drop on Q1 and design choice of  50'C rise max. @ max ambient,  Pmax = 250 mW at ~21 mA, Although 36.1 °C/W heatsinks exist.

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The process to choose 12V requires knowing;

1. Zener is 6.2V @ 5mA test current, so using less reduces voltage slightly. (knee resistance ...  R*I)

2. Q2 collector current depends on Vbe (quick guess if Ic= 1mA, Vbe = 600 mV)

3. Vbe depends on Thevenin voltage Vth from R divider from 12V   and Vz on emitter.

4. The R values minimize current may be computed several ways such as derating hFE min by 50% for cold temperatures for a stable result.

however the quickest way for me would be to simulate in a few minutes for error=2% max.

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If I was given a fixed load of 1K, then I would simplify as follows.  with 180 uA no load @ 12.18V\\https://tinyurl.com/2y9sa555