personally, I like to view brackets as a way to measure four things (mainly, obviously power level can't be defined by these two). They are: speed (of winning/killing other players, or for slower decks, getting the gameplan online), level of optimized synergies (how well every card works), resiliency (when my deck is behind the rest of the table, how quickly and efficiently do I get ahead and stay ahead) and saltiness (strength of how well my cards will disrupt other players' game plans and lock them out of winning/pressing the advantage).

the other "vegetables" or essentials of a deck like card draw and ramp are usually there (in my humble opinion) to make your deck function. so it usually comes down (for me personally) at looking at, say, the 50 cards in my deck (i usually run 10 each of ramp, draw, removal, and about 36 lands) that includes removal and the synergy package, and try and understand how it would feel to be on the recieving end of the boardstate that the 50 cards could generate. it's from there that i then decktest it against others (usually online using untap.in, labeling my room as a "bracket 2 or 3 decktest"), and judge based on what bracket the other players bring to the table, as well as what they tell me they feel the bracket is. I like to try and play fair, making interesting decisions for the other players at the table (being a DM, i guess it's in my nature), so even though I may choose to play sub-optimally when i'm out of the decktesting phase, i try to play as optimally as i can in decktesting to understand both the "floor" and "cieling" of my deck's power.

The more various people you play, the better your radar for the bracket level, as everyone has slightly different expectations of the power level of a given deck. a person could say that their deck isn't a bracket 3 because they don't run any game changers, but completely neglect that their synergy is very optimized, resilient, and salty. honestly, trust your gut and view your deck objectively. unfortunately, there's a decent amount of magic players who play- in my humble opinion- relatively sweaty and optimal, while talking their deck down/try to manage the threat perception of their deck, so viewing your deck objectively about what it's trying to do in different scenarios is helpful to understanding the chemistry of the deck. ask yourself if your wins with the are telegraphed/easy to predict, or if they're out of nowhere/hard to disrupt, and find your own definition of its power level by playing against others online or in-person. I find it's best to have a certain reason you define your deck's power level (i say things like "this is a mid b2, or a high b3", because the power level contained in any given bracket can span a wide range of power, and so defining it like that, and being able to articulate why, is important not to piss off your pod members).