You can use a 30k pot (I've never seen such a beast) but you need to increase R3 to 22k if you want to use it.
I wouldn't suggest a higher value, I'd recommend going lower, do you have a 2k or 1k resistor?
You can use 30k if you like but it's bit high because of the loading created by the base current.
The regulator works because the base-emitter voltage is roughly equal to 0.7V and the diodes drop 0.7V each (1.4V in total).
The base voltage of Tr1 is varied between just under 0.59V to 1.4V as the pot is adjusted, the emitter voltage will always be 0.7V below the base so will vary between 0V and 0.7V.
Because the voltage across R2 is kept constant Ohm's law that the current through it remains constant so the current will vary between 0/33 and 0.7/33 or 0 to 21mA.