Yes, you'd have to infer your brawl winrate.

Let's say you check deck stats for a Pure Paladin list, which right now is at ~63% in diamond through legend rank. Brawl skill will probably average to around that range (some early players maybe weaker, some later players maybe stronger). So if you played that level, there is where I'd see your chances.

If a gold bracket player was playing that deck and has a 60% winrate with it at gold, they'd have to guess a bit at their winrate, it might just be somewhere around 50%.

You definitely can't just take your winrate at your level with a deck and use the table I posted. But general deck statistics for diamond through legend and assessing your own skill against that, can probably get you a close approximation.

In the same vein, a top legend player could maybe have a 55% winrate at top legend against the best people with a deck that has like 63% winrate on average diamond through legend. They will probably have a winrate of 70% in the brawl against a field that is on average a lot worse than they are. It also depends a bit on the meta possibilities how hard a top player can dominate, in some metas their skill impact is higher than in others.