Available trading amount and a little bit about my strategy.

I’ve recently started selling naked puts, and I’ll be honest, having a blast so far. Unrelated to my question, but my strategy is pretty simple (and surely not unique). I sell on TQQQ with an expiration no longer than 10 trading days away, usually 5 though, with a strike price 70% of current market value or lower. I can usually bring in $0.01-$0.05 premium, but during volatile weeks like this one, closer to $0.10.

Anyway, now to my question. I hold almost no cash in my account, and my original available funds for trading was about $11,600. I sold my first put at a $42 strike and my thought was that it would go down to $7,200. I thought it would take off the $4,200 I’d need to cover the position if it was exercised. But rather, it brought it down to about (I forget the exact number) $10,500 give or take. I’m trying to understand why?

Tried my best going into selling naked puts with my eyes wide open, and although I do feel confident on my understanding, this sure threw me for a loop. I’m not too upset because I way over estimated, but I want to know how I can figure that number myself because I’ll honestly likely write more contracts if I’m able to. Originally thought I would only be able to write 2 or so at a time.

Sorry for the long post, but I appreciate your help, and I’m happy to answer any questions.

At Schwab btw.