The very purpose of setting a high number of NuShares required for minting is so not everyone can vote.
As in https://nubits.com/nushares/introduction
This limit is in place to prevent blockchain clutter from individuals without a vested interest in the network, while still allowing small shareholders to offer input into network decisions.
The time will come when this number has to be changed. But given the way NuShares have been distributed, it is certainly not yet.
Aside from that, I think it should be calculated from the blockchain data and a predefined voting-shareholder percentage (maybe set by a motion?).
Lets say we want the 60% of shareholders to vote, we scan the blockchain and read the lowest balance of the 60% richest addresses and there we have the new minimum. Of course this adds the risk of people splitting their shares between multiple addresses and generating “virtual shareholders” for the calculation.
These are some rough ideas, but I think the minimum value should be somewhat based on blockchain data.