Since you will want a maximum flow that is comparable to an existing typical gicleur, which is 0.7mm and you'll also want to have the ability to effectively stop the flow, you'll need an orifice much larger than a typical gicleur to achieve both end conditions within a half turn of the valve.
The COTS valve I'm using is a 0.070" orifice and a 15° taper with micrometer threads. Based on the published Cv
curve for the valve and the calculated Cv
of my 0.5mm gicleur based on measured water debit numbers, this geometry should provide what we're looking for. It is also available in 5mm as well as 10-32 threads, so it should conveniently fit behind the mushroom cap when threaded into the mushroom in place of the gicleur with an appropriately threaded bushing.
I have 18mm from the "floor" where my gicleur threads in to the face of the sealing washer for the cap.
Just food for thought,