One possible (and better) way to do it would be a linear scale.
100K balance (or less), margin would be regular day trade margin.
101K balance, margin would increase to 101% of the regular day trade trade margin.
..
120K balance, margin would increase to 120% of the regular day trade trade margin.
..
150K balance, margin would increase to 150%
..
200K account and above, margin would increase to 200%
This way you can always trade the same number of maximum lots, and don't have the odd situation where you suddenly lose 50% of your buying power when the account goes a bit above 100K.
This should also be easy for the AMP team to implement.
You need to remember something here. Discount brokers like AMP are doing y'all a favor by allowing these reduced "day-trade margins".
Day-trade margins do not exist at the CME. They are provided to the clients of the FCM as a courtesy by said FCM, like AMP.
The ES currently has an initial margin requirement of $12,100 per contract.
When you are trading your one-lot on the ES with a $1K account at 400 bux "day-trade margin", how is that possible? It is because a firm like AMP is LENDING you the extra $11,100 from their seg account funds for you to initiate and hold that position.
Now. With their generous loan to you along those lines, and you go ahead and trade your $100K account gobbling up all that discount, what is happening?
Let's go max lever.
$400 into a $100K account? They are allowing you to trade 250 ES contracts on margin. So you have donated your $100K. How much money does AMP have to put up to maintain your position for the day? $2.925 million.
That is what THEY have to use from THEIR end, along with your measly $100K, to meet the exchange requirements for that performance bond.
When the settlement time comes, if you do not have the extra $2.925 million in your account, (aside from the actual position's PnL), they will liquidate your positions, whether in profit or loss.
From what I recall for an overnight hold, they must provide 8% above and beyond your total NAV in positions.
So if you are at BE NAV by settlement time/market close, they still have to provide 8% of that 3 million of your account to the exchange.
With that in mind, you can see how unwieldy it can get.
Think about it, and stop staring at the gift in the mouth of the horse.
In the famous words of Abashi the Sock Puppet, "Quit Yer' Bitchin!"