I agree that big margin calls have a significant impact on the market, there is room for improvement.
Are there any mechanics in the fiat world to learn from / compare to?
Anyways, I like your approach, but.it.seems overly complicated, but maybe there is thought in it that I can't grasp yet, I am in no position to qualify your proposal. Could you explain more why you wanted two parameters?
Anyways, the first thought that came to my head is to introduce a fallback collateral ratio (can be set optional by the user when creating the position, to not break existing logic, must be e.g. 1.85 or higher) . The mechanic would be as follows:
If a position is margin called, an order is placed to sell as many BTS as needed to reduce the debt, such that the position has a collateral ratio higher or equal to the fallback ratio after the sale.
This introduces also a implicit reserve and enhance part but utilizes only one parameter.