1%-49%-at the risk of being repetitious to drive home an extremely important point, dont put your firm in a situation where your three biggest clients are responsible for more than 50% of your firms monthly service revenue.
if any of these clients are aware of this control they have over your monthly numbers, it gives these clients too much leverage...especially for jerks who choose to abuse this power. but even when that information is successfully kept under wraps, its just too risky for your firm to be in situations where the loss of one or two of your biggest clients could potentially put your company out of business.