How to lower total retention even more...
Idea is very simple, to fill the dead space with properly designed adapter.
Adapter is made from aluminium alloy, hand pressed on original upper burr carrier and securely bonded with Loctite 638.
I measured total retention after modification - cleaned the grinder chamber, chute and spout with vacuum cleaner, then single dosed 10.00g, output was 6.63g, so total retention is only 3.4g (!), reduced from original one by 2.5g. Practical retention will be even lower.
From earlier brush cleaning some time before chute itself holds 2.3 - 2.4g of coffee,
so I plan to try some modifications to the flapper (i.e. to make a new one).