I know not many here have hands on experience with the GRA gear but I'm looking for a little advice. @storm, I'm look at you

I played with and raced LPG powered vehicles using Impco equipment many years ago and always wanted to experiment with a turbo setup.
It took 30 years but I'm finally doing it!
I've just bought a car fitted with a 202ci Holden inline 6, GT3082R and a GRA s350 mixer.
There's not a lot of info available as the heyday for similar setups was 20 years ago or more. I've only found a few posts on o-l-d forums regarding an off idle stumble and none has been of much help.
In my case the motor starts and idles well, but if I crack the throttle slightly there's a dead spot where the motor will stall.
The only way I have found to eliminate it is to use a progression hose with no jet fitted. (results in an idle of 0.9-0.95 lambda). So I know it wants more fuel just above idle and the wbo2 confirms this as it goes way lean when i touch the throttle.
Of course the open progression hose screws up the AFR's elsewhere else.
The documentation says the idle fuel orifice is factory set at 1.2mm. Mines at 1.35mm, an area increase of 26%, so I know someone has been here before.
My first question refers to the position of the progression slot vs the throttle blade at idle. My p.slot is exposed more than I would have expected, so it appears the motor is needing a bit of throttle opening to idle. This throttle opening is pulling fuel from both the idle jet and a good percentage of the p.slot.
I'm wondering if I should be drilling the idle gas port, closing the throttle to cover more of the progression slot (to make more slot length available for cracking the throttle), but I'm not sure if there's enough air available via the adjustable idle air bypass to idle and close off the throttle? I'm thinking this will make the progression slot more active when cracking the throttle?
Am I on the right track? don't want to be drilling out the idle jet unnecessarily.
Thanks in advance!