Proposal Threshold and Voting Quorum: percentage of the total supply vs fixed value

[franckc]

I'm looking at setting up a Snapshot space that uses a governance token with a circulating supply that varies significantly over time. Therefore, as opposed to using fixed values when configuring "Proposal Threshold" and "Voting Quorum", I'd like to set those as percentages of the total supply. For example to enforce that the author of a proposal has at least X% of the total voting power.

1. What would be the best way to do this for "Proposal Threshold"?
   Is there anything existing that I could re-use? Or would you recommend to write a new custom validation plugin?

2. Same question for "Voting Quorum"?
   Is there anything existing that I could re-use? Otherwise, is there a way to write a custom plugin for quorum evaluation?

Thank you!