Bearing in mind that it's possible that 1 chevron = rounds to 1 rather than exactly 1 and that systems don't have identical starting values, which might mean it's not exactly the same from system to system, generally 8-9 seems sufficient for high security.
Note that a lot of BGS states affect the security level, generally downwards, so "barely in high security" means "medium security while there's any of those states" (which may be what you're after, of course!)