That's not the jets (as such) it's the fact that when you lift the pedal, it's less volumertically efficient and less likely to detonate a less dense, lower volume, less compressed intake charge. Just the same as a non SC engine (*with locked timing) will detonate (eventually) at full throttle but not at part throttle.

You'll reach a point where reduced timing just isn't going to cut it, you'll need less compression, less boost a cooler charge or higher octane fuel (and of course most of those are inter-related).

If you go to a paint wholesaler, ask for toluene (a paint solvent) also known as 'methyl benzine' . Make sure it's a pure source, not mixed with anything else. Run about 30% toluene to 70% petrol (highest octane pump fuel) that's about the limit of the toluene concentration before it starts to cause fuel lines and other stuff to deteriorate. And flush it out (run to empty on the mixture, then re-fill with petrol) with pure petrol if you wish to leave the car sitting for a few months.

Even cheaper - get the existing washer bottle, and aim the spray nozzle on the primary barrel of the carb, the venturi, throttel plate and even the supercharger itself will help break it up - try manually spraying it and see if the pinging disappears on the road at full throttle. It probably will.