Concentrated, but not dangerously
46% with one carrier$1.5M of premium and about $188k of annual commission sit with your largest carrier. A rate action, an appetite change or a non-renewal decision taken in a room you are not in moves that much of your income. This is the single most common reason an agency looks at a network, and not because the current carrier is bad. It is because there is no second door.
What to check
Pull premium by carrier for the last three years and look at the trend rather than the snapshot. Then check whether your carriers are genuinely different bets: three carriers all heavily exposed to the same class or the same geography is concentration too, even though it looks diversified by name.
What would move it
Do not rewrite accounts you are happy with to hit a ratio; that is expensive and it annoys clients. Direct new business elsewhere instead, which lowers the percentage by growing the denominator. And get the second market appointed before you need it, because the worst time to go looking is the month your main carrier exits.
Show the arithmetic
index = largest² + (n−1) × ((1−largest)/(n−1))² = 0.270, with 6 carriers. Below 0.18 is well spread; above 0.25 is concentrated.