Any rules of thumb; widely used or from experience for ceilings on customer concentrations for top 1, 5 and 10 customers? I'm reviewing deal with 25%, 40% and 75% respectively generally with 20 year tenure with company. I know it depends on contracts, exclusivity, moats, nature of service or product, competitive alternatives....but just rule of thumb. This one strikes me as 'high' but maybe borderline for the right price.