There's a threshold at 50% and a threshold at 90%. The common wisdom used to be a BT at 89% of credit limit to totally max out an account, but that was back when interest rates made it worth it. I wouldn't go over 49% these days.
I'd recommend doing a security freeze if you're going over 50%, you'll still probably lose the accounts with the bank you're BTing from, but at least the others won't see it.