The 1800 BTC loss made up 20.96% of clams, if they were bought at the high. In other words, you could spend 1800 btc to buy 20.96% of clams outstanding at the high price. At 2.5x leverage, Poloniex's customers need to have collateral valued at 40% of margin loans outstanding, calculated by x = 1/2.5. 40% of 20.96 is 8.38, so an additional 8.38% of clams outstanding would need to be used as collateral for an 1800 BTC margin loan, bringing the total to 29.34% of clams.

I believe the above is actually how the margin system still works on Poloniex, and this actually results in traders being able to get 3.5x leverage, because they can use 1 BTC to buy 3.5 BTC worth of a position if the collateral is the same as the trading pair. I was able to replicate my mistake in saying that 35% of clams outstanding were used in margin loans plus collateral by incorrectly assuming a total of 2.5x leverage, or borrowing 1.5x the value of the clam collateral, or having equity of 60% of the margin loan.