How do I buy max amount possible with the remainder?

Hi all.

So, in my paper trading account, the initial buying power is $400,000. All I want to do is to buy, let’s say, SPY stocks, on all stake. Right now the price is approximately $294, which is ~ 1360 shares available to buy, but… the real amount possible to buy is 1325. Turns out the SPY price is 301? It just can’t be, right? How do I buy max amount possible with the remainder?

Thanks in advance.

I believe you are buying on margin when trying to use the full 400k. The docs on maintenance margin should explain why you can’t use all 400k.