Class PX Current Transformer knee point voltage sizing questions

[electrical429]

I know that CT knee point voltage is calculated as follows:

Vkp = K * If/CTR * (RCT + RL + RR)

Where,

K = Constant, conventionally taken as 2.0
Vkp = The minimum Knee Point Voltage
If = Maximum Fault Current at the location, in Amperes
CTR = CT Ratio
RCT = CT Secondary Winding Resistance, in Ohms
RL = 2-way Lead Resistance, in Ohms
RR = Relay Burden, in Ohms

Questions:

1. Is it correct to say that for numerical relays RR would be ~ 0 Ohms? Most of the data sheets I've seen state CT input burdens in the region of 0.1 VA.
2. What would be the disadvantage of using higher ratio Class PX CTs? As per above formula the higher the CT ratio the lower the required knee point voltage.
3. Most of dual ratio Class PX CT specs I've seen state only one knee point voltage, and some stated same knee point voltage for both ratios. I assume that for dual ratio CT, knee point voltage for lower CT ratio should be two times higher than knee point voltage for higher CT ratio.