/diy/ - Do It Yourself

I'm trying to design a flyback transformer for a Geiger counter, I've been playing with this 1:10 (26:260) flyback I've made, trying to clear out basic concepts, it kinda works fine stepping from 5V to 50V, but I still have many doubts.
The mode of operation will probably be DCM, since the output current is tiny. I know the output is roughly Vin*(N2/N1)*DutyCycle, but I've still got to figure out the number of turns in the primary, the primary inductance, the air gap required, and apply concepts like permeability. Any rundown or recommendation on how to design this stuff? I'd probably pick up a feedback controlled variable duty cycle driver once I've got the basics.

Also, what's a good transistor that can switch a decent load at 4 volts Vbase/Vgate?