I've allocated $330 of my BR to playing S&G tourneys. The rest of my BR is allocated toward ring games (which I'm taking an extended break from) and buffer.

I'm currently playing an even mix of $5.50 and $11 tourneys.
When I get to $440, I'll drop the $5's and go exclusively $11.
When I get to $660, I'll go to an even mix of $11 and $22.
When I get to $880, I go with exclusively $22.
$1100 mixes $22 and $33 (assuming $33 exists)
And so on.
Now for a more thoughtful approach, maybe...
The players at the $5's and $10's are exactly the same, not just in skill level, but the same people. So the play is clearly no more difficult at twice the buy-in, but the rewards certainly are twice as great. Is my $330 BR enough to handle the potential downswings?
Let's say I go 10 games without an ITM finish (current long cold streak is 5). That's $110 out of my BR, taking me down to $220. I can handle that swing, but any more than that would have me not only questioning whether I should move down, but also whether I should move out. So I think maybe I could play exclusively on $10's now. Reasonable?
But if I can play the $10's now, when can I play a mix of $10 and $20? That depends on the level of competition, I suppose, but let's assume it's slightly tougher. Clearly I can't tolerate a 10 game downswing at my current BR, as I'd be down to $110 and very uncomfortable. So, at which point can I tolerate a $220 downswing?
If I can accept losing 1/3 of my BR to a 10 game downswing, the number is $660.
So there's my formula: I should be playing at a level where a 10-game downswing will cost me no more than 1/3 of my BR.
Is that reasonable?
With that new formula, I should do...
$11 buy-ins at $330 (now)
Mix $11 and $22 buy-ins (average $16.5) at $495
$22 buy-ins at $660
OK, this is looking pretty clearly like 30x buy-in for the BR requirement. Simpler formula.
As long as I keep my ITM% upwards of 40% (currently mid-40's) and my ROI% upwards of 40% (currently high-40's to low 50's, small sample size so each win swings it wildly), I should be good to go, yes?
What's conventional wisdom re BR size to buy-in?