Thank you for good input.
Yes have noticed how WSBK riders compress forks and they STAY compressed in bottom 1/4th stroke while transfering brake load to cornering lean load.
Guess tuning that zone is very important to them.
Just looked up that K-tech recommended level settings for RDS (conventional) and DDS (pro level) are 145mm and 185mm respectively. Rod diameters (defining airgap volume) are about same, but still 40mm difference!
In this particular instance you are comparing apples to oranges, let me explain. The RDS is a open bath cartridge, so oil runs from inside the cartridge to the outside. In order for this to work the entire cartridge MUST be submerged in oil otherwise air will be sucked into cartridge and damping will be killed. In addition the oil level is setting air gap. So for an open cartridge (Ohlins 30mm, K-Tech RDS, GP suspension) the oil level serves two purposes, air gap, and making sure the cartridge is fully submerged.
The DDS cartridge is a fully enclosed pressurized system. What this means is the oil never leaves the cartridge and always stays full. This allows you to run extremely low oil levels without introducing air into the cartridge. So in this case when your setting the oil level you are ONLY using it for air gap unlike open bath cartridges.
You mentioned rod displacement, in this conversation rod displacement does not have an effect like you would think. In a pressurized system the rod displacement will not change the oil level height in the cartridge. In open bath like the RDS it would be minimal level, not enough to even worry about.
EXAMPLE - Anthony West this weekend at PittRace was on K-Tech DDS and Dave Anthony was on K-Tech RDS. We wanted to run 210mm on Anthonys forks but could not because we were too close to the top of the cartridge, so we ran 190mm. On Anthony's bike we have no issues so we can run 210 to 220mm all day long without any issues.