Trying to build a BR playing 6 handed SnG's

If you play 6-handed, you'll get less callers for your monster hands so you'll hardly ever be an above-average stack.

I guess if your post flop play is spot-on, you won't get any floaters.

Most long-term SnG players (myself included) use a much more conservative BRM model than 5% / 20 buy ins. This is because of the super-swingy nature of the games: streaks where you don't cash for 10, 15, maybe even 20 games at a time are pretty standard regardless of how good a player you are, and when they happen if you're using the 5% model you risk wiping out almost all of your roll.

A lot of regular SnG players prefer to have at least 50 buy ins for the level they're playing and I'd probably consider that to be the minimum. Personally I'm a bit of a BRM nit and prefer to have 100 or more.