Switching Long to Short, Different Margin Multiple?

Long trades always work… I just take my buying power, put in a notional market order for the whole amount, and it buys as many shares as it can with it. Trading short gives me no end of problems. First, it seems we can’t do notional with short trades (boo), and so I have to math out how many shares I think I can swing with a market trade. I’ve tried doing buying power / share price, but of course share price is fluid and by the time I get my order in I get “insufficient funds”. Fine, so I back off and I only do (buying power * 0.98) / share price… that seemed to work. I’ll pause here…

Question #1: When trading short, I’d like to uses all my buying power. Is there a better way to trade short that Alpaca will math things out? Am I correct that notional trading short is still not a thing? Curious what recommended practice might be.

Ok, moving on to my latest issue. I’m flagged as a pattern day trader, so I get 4x multiplier on margin. However, today it wasn’t giving me the full 4x when I sold my long position and attempted to open my short position. Kept giving me this error (excerpted), even though my base equity was in the ~30k range, I should have had ~120k of buying power, and clearly my algorithm when it queried my account for my buying power got the answer that I DID have that, but then when it attempted to put in the short trade, it said my buying power was only ~$96k.

Question #2: Is there some secret about short selling buying power… or selling a long trade straight into a short trade…?? I’m sure I’m just ignorant of some secret here.

data: {
  buying_power: '96866.4',
  code: 40310000,
  cost_basis: '116106.96',
  message: 'insufficient buying power'

I should mention, I was trading TSLA, so I don’t think any of the “leveraged ETF” rules should apply (that would be weird)

…and, this wasn’t overnight, this was intraday trading, so the 30% maintenance margin shouldn’t have applied, right?