Is the Lagom P64 really zero retention as claimed? Do you need to RDT*?

Yes, we have done many iterations to arrive at the current design that minimises deadspace and surfaces that coffee grind can cling to. In typical use, the retention is below 0.1g without RDT. We have found that RDT is optional after break in and there is no retention for most coffees except for some dark-oily roasted coffee or drier coffees. There may be up to 0.3-0.5 g of coffee built up in the grinder over the long term use but these do not come off in regular use and will not affect the taste of your coffee.

*RDT is the technique of applying a small amount of moisture on the coffee beans before grinding as a way to mitigate static electricity generated by coffee-grinding.