Forgot to answer this earlier, but:
For rough sawn stock I calculate the the rough sawn board requirement that will yield the finished dimensions. For example, ten pieces at a finished size of 800 mm X 44 X 18 (31-1/2" X 1- 3/4" X 3/4") will come out of ten pieces of rough timber 36” X 2” X 1” = ~0.42 cubic feet. Add an additional waste factor even though you've calculated using fairly generous board sizes, so for waney edged English oak use an additional waste factor of 100%, so calculate 0.42 ft³ X2 = 0.84 ft³. Typical waste factors for rough sawn square edged boards are 50% for walnut and 30% for cherry, and 20% for poplar, ash, beech, etc: sapele generally fits within the ~20% extra category because there tends to be not a great deal of waste.
Accoya, as others have said comes in pre-machined sizes of length, width and thickness so it's essentially just a case of working out what standard width and thickness you can work with and what you can get out of the standard lengths available. Typical standard lengths range from 2.4 m up to 4.8 m long. What's available between 2.4 and 4.8 m length depends upon your supplier. Slainte.