This is an issue common to all types.Most Take off performance charts have a range that at the top end exceeds the structural limit weight, and at the lower end often has a lower limit that often can be higher than an operating weight with little or no payload, ferry flights for example.
Some companies simply say "use the lowest chart weight " for a specific runway if your ACTUAL weight is lower than the chart, however these speeds might then be significantly higher than FCOM speeds for the actual weight.Obviously performance (engine out0 is not an issue but it is quite acceptable to refer to the FCOM/QRH to obtain the correct speeds for your actual weight on the day, as this has the effect of avoiding a deliberate "overspeeding" during the time on the runway.Pete.