The zener current is determined by the voltage drop across resistor. This is not the supply voltage, it's the gate-cathode trip voltage. In your example, that's 1 volt, so the resistor should be 1v/8ma=125Ω. The nearest standard value is 120Ω.You have a max of 300mA output current ?
if so, these components are way to much heavy dutty for that!
There are SCR's with a Holding Current(IH) of less than 10ma , you will need a resistor that allows that and a Zener with a Izt equal or greater..
example : TIC 106 (5amp SCR), IH of 8mA, 1volt triger
zener BZX85c33, with Izt of 8mA
resistor : R(max) = 30v/0.008 R(max) = 3750, near EIA e24 code = 3300ohm
capacitor : more bigger the more stable versus delay it gets
this is just a cheaper option, do you agree CDRIVE ?